In the final days before the United States faced a full-blown epidemic, President Trump made a last-ditch attempt to prevent people infected with the coronavirus from reaching the country.

“To keep new cases from entering our shores,” Trump said in an Oval Office address on March 11, “we will be suspending all travel from Europe to the United States for the next 30 days.”

Across the Atlantic, Jack Siebert, an American college student spending a semester in Spain, was battling raging headaches, shortness of breath and fevers that touched 104 degrees. Concerned about his condition for travel but alarmed by the president’s announcement, his parents scrambled to book a flight home for their son — an impulse shared by thousands of Americans who rushed to get flights out of Europe.

Siebert arrived at O’Hare International Airport in Chicago three days later as the new U.S. restrictions — including mandatory medical screenings — went into effect. He encountered crowds of people packed in tight corridors, stood in lines in which he snaked past other travelers for nearly five hours and tried to direct any cough or sneeze into his sleeve.

When he finally reached the coronavirus checkpoint near baggage pickup, Siebert reported his prior symptoms and described his exposure in Spain. But the screeners waved him through with a cursory temperature check. He was given instructions to self-isolate that struck him as absurd given the conditions he had just encountered at the airport.

“I can guarantee you that people were infected” in that transatlantic gantlet, said Siebert, who tested positive for the virus two days later in Chicago. “It was people passing through a pinhole.”

The sequence was repeated at airports across the country that weekend. Harrowing scenes of interminable lines and unmasked faces crammed in confined spaces spread across social media.

The images showed how a policy intended to block the pathogen’s entry into the United States instead delivered one final viral infusion. As those exposed travelers fanned out into U.S. cities and suburbs, they became part of an influx from Europe that went unchecked for weeks and helped to seal the country’s coronavirus fate.

Epidemiologists contend the U.S. outbreak was driven overwhelmingly by viral strains from Europe rather than China. More than 1.8 million travelers entered the United States from Europe in February alone as that continent became the center of the pandemic. Infections reached critical mass in New York and other cities well before the White House took action, according to studies mapping the virus’s spread. The crush of travelers triggered by Trump’s announcement only added to that viral load.

“We closed the front door with the China travel ban,” New York Gov. Andrew M. Cuomo (D) said last month as officials began to grasp the magnitude of the failure. In waiting to cut off travel from Europe, he said, “we left the back door wide open.”

Trump has repeatedly touted his decision in January to restrict travel from China as evidence that he acted decisively to contain the coronavirus, often claiming that doing so saved more than a million lives. But it was his administration’s response to the threat from Europe that proved more consequential to the majority of the more than 94,000 people who have died and the 1.6 million now infected in the United States.

White House officials noted the president was widely criticized for the move to limit travel from Europe, with many saying it was too draconian at the time. “The president took bold, early action that I think few leaders would be willing to take — and because of that he saved countless lives,” spokeswoman Alyssa Farah said.

The lapses surrounding the spread from Europe stand alongside other breakdowns — in developing diagnostic tests, securing protective gear and imposing social distancing guidelines — as reasons the United States became so overwhelmed.

The travel mayhem was triggered by many of the same problems that plagued the U.S. response to the pandemic from the outset: Early warnings were missed or ignored. Coordination was chaotic or nonexistent. Key agencies fumbled their assignments. Trump’s errant statements undermined his administration’s plans and endangered the public.

“We kept foreign nationals out of the country but not the virus,” said Tom Bossert, who served as adviser of homeland security at the White House until last year. The move to restrict travel came when it was more urgent to arrest the spread of infections already in the United States, Bossert said. “That was a strategic miscalculation.”

This article tracing the administration’s response to the Europe threat is based on interviews with dozens of current and former U.S. officials, as well as public health experts, airline executives and passengers. Some spoke on the condition of anonymity to offer candid assessments of events, decisions and internal administration debates.

An agitated president

The Europe restrictions, which remain in effect, bar entry to non-U.S. citizens or permanent residents from 26 countries. Britain and Ireland were at first excluded from the list before being added on March 17.

The decision came at a time when the country was still resisting other measures critical to containing the outbreak. Schools remained open, states were not yet issuing stay-at-home orders, and many officials were still emphasizing hand-washing as an adequate means of preventing infection.

The lack of urgency was driven by a failure to understand the threat’s true dimensions. There were only 3,714 confirmed cases in the United States on March 13, the day the travel restrictions were implemented, and just 176 deaths had been recorded. Those numbers are considered woefully inaccurate, artificially suppressed by the scarcity of tests.

Within days, Trump would assert that he grasped the full magnitude of the danger soon after the virus escaped Wuhan, China. “I felt it was a pandemic long before it was called a pandemic,” he said on March 17.

Yet Trump spent much of the preceding month predicting the virus would quickly recede and downplaying its severity. “It will go away,” he declared on March 10, one day before his address from the Oval Office. “Just stay calm. It will go away.”

Behind the scenes, senior officials had been agitating for weeks to consider expanding travel restrictions beyond China. Deputy national security adviser Matthew Pottinger, who had been based in Beijing as a journalist, argued during meetings in February that transmission was higher than being reported in China and that if community spread began in Europe there was little prospect of containing it.

Pottinger made the case that “once it was in Europe, it was going to go ‘whoosh,’ ” a senior official said. Members of the administration’s coronavirus task force were even presented with charts showing that the number of flights arriving from Europe dwarfed the influx from China.

By the third week in February, the fears about Europe were becoming reality. On Feb. 22, Italy issued quarantine orders on 11 municipalities in the northern part of the country. It closed schools, canceled public events and halted train travel in the same region. Because there are no constraints on crossing borders within continental Europe, the developments in Italy meant that spread into other countries was inevitable.

But Pottinger and a handful of other officials who shared his concerns faced opposition from powerful administration figures fearing enormous economic fallout. Among those arguing most vehemently against curbing travel from Europe, officials said, were Treasury Secretary Steven Mnuchin and Larry Kudlow, the president’s chief economic adviser.

Even health experts at times seemed skeptical. Anthony S. Fauci, the nation’s top infectious-disease expert, at first reacted skeptically to limiting travel from Europe, saying in a February meeting in the Situation Room that the available data did not support such a move, the senior official said. A spokesperson for Fauci declined to comment, referring questions to the White House.

Few countries were then imposing travel restrictions on nations other than China and its neighbors in Asia. Europe did not issue comprehensive travel restrictions until after the United States had done so.

Debate on the issue was also derailed by turmoil on the coronavirus task force. Trump put Vice President Pence in charge of the panel on Feb. 26 as Italy confronted a surging outbreak. Officials said it took a week or more for Pence to get up to speed on the threat and array of possible responses.

Serious deliberations about Europe didn’t resume until mid-March. By then, Pottinger had gained a new ally. Deborah Birx, who had joined the task force earlier that month, entered a White House meeting armed with worrisome data on a surge in cases in northern Italy, as well as numbers that showed accelerating spread across Europe. Then, on March 11, the World Health Organization declared the coronavirus a global pandemic.

A tense meeting of task force members and other White House officials followed that afternoon in the Cabinet Room. A small contingent then gathered around Trump in the Oval Office.

Mnuchin remained against the move, officials said, vociferously arguing about its potentially damaging effects on the economy. But others present, including Robert C. O’Brien, the national security adviser, and Alex Azar, the secretary of health and human services, argued the United States could no longer justify the risk of allowing travel from Europe to continue unimpeded.

Trump sided with the majority. But the magnitude of the undertaking — constricting one of the busiest air travel corridors on the planet — seemed to escape him. And the logistical requirements of implementing this plan on a 48-hour timetable were not even meaningfully discussed, officials said.

Instead, Trump and his inner circle seemed focused on staging the announcement for maximum political impact, officials said. Jared Kushner, the president’s adviser and son-in-law, urged Trump to deliver a formal speech that evening and argued that the details should be kept close-held to prevent them from leaking.

Kushner then gathered with senior policy adviser Stephen Miller in the latter’s office to work on a draft. The duo were joined at times by Pence and were still making edits until shortly before Trump was scheduled to go live on television at 9 p.m.

No drafts were shared in advance with members of the task force or any of the agencies that would have to carry out Trump’s decision, officials said.

“The president was in a bad mood,” one official said. As he settled into his chair, Trump cursed about a stain on his shirt. “He wasn’t convinced the speech was a good idea.”

It was only the second Oval Office address of his presidency, reflecting the gravity of the moment. But the result was a stumbling performance in which Trump struggled to follow the text on the teleprompter and committed a series of gaffes.

“Never has a less prepared set of remarks been delivered from that room,” said a former administration official.

The actual policy included no plan to cut off cargo shipments between the continents, for example, but Trump indicated otherwise. The restrictions “will not only apply to the tremendous amount of trade and cargo,” he said, “but various other things.”

The new restrictions included “exemptions for Americans who have undergone appropriate screenings,” he said. But few caught that important caveat after his opening declaration that the United States was “suspending all travel from Europe.”

As networks cut away, Trump was caught muttering a drawn out “okayyyyy” as he slumped in his seat. Afterward, he groused about his performance, officials said, while subordinates issued statements and tweets to clarify or correct his misstatements. Within days, he was blaming Kushner, telling aides that he shouldn’t have listened to his son-in-law.

Racing to get home

Even the timing of the speech turned out to be ill-considered. It came at the tail end of a three-hour window during which dozens of red-eye flights depart the United States each night for cities across Europe. As a result, thousands of passengers learned about the new policy while over the Atlantic and scrambled upon arrival to alter their plans.

At Dulles International Airport outside Washington, the cabin door on United Flight 989, headed for Frankfurt, Germany, had just been secured when Trump’s speech began airing on television networks. As he spoke, passengers began rising from their seats in panic. Brandishing bulletins about the speech on their cellphones, some pushed for the exits.

“He said they’re closing the borders,” one passenger said. “I want off this plane.”

The pilot and cabin crew began making frantic calls to supervisors for guidance. Bobbie Mas, a veteran flight attendant, dialed a hotline for United employees, then the company’s staffing office at Dulles, but no one had answers.

She then entered the cockpit to speak to the captain, who would be first in line for any major air travel advisories. The captain contacted United’s operations desk — the nerve center of the airline — but officials there were similarly scrambling for details.

The only warning conveyed to the airline was a call that United’s then-chief executive, Oscar Munoz, got from an administration official “literally minutes” before Trump began speaking, a company spokesman said. The official provided no details about what Trump would be saying except that it pertained to air travel.

By the time the Boeing 777 departed for Frankfurt two hours later, nearly every U.S. citizen had gotten off the plane. For many, the decision was driven by the erroneous impression created by the president that they risked being stranded in Europe for a month or more.

Among those who deplaned was Mas, who is also a union representative with the Association of Flight Attendants. Worried that she had not packed enough prescription medicine to last a month trapped in Europe, she said she asked to get off an aircraft for the first time in her 21-year career.

“There was fear and chaos,” she said. Save for the tense days that followed the Sept. 11, 2001, attacks, she said, “I have never seen anything like it.”

Even more chaos was in store.

Airlines’ websites and phone lines were inundated in the hours after Trump’s Oval address. American Airlines fielded about 700,000 calls on March 12, a spokesman said, more than five times the number on a typical day.

Travel across the Atlantic surged. The number of passengers arriving from countries targeted by the restrictions soared 46 percent in a single day, up from about 31,000 on the day Trump delivered his address to 45,399 the next, according to data from Customs and Border Protection. Friday’s traffic was even higher, topping 46,000.

Many were U.S. citizens racing to get home before midnight March 13, when the restrictions were scheduled to take effect — unaware that they were exempt from the policy and faced no deadline. Even when given accurate details on the policy, many refused to put off their travel, fearing the administration might abruptly switch course and end the exemption.

One airline industry official, who spoke on the condition of anonymity, said that gate attendants at multiple airports began making panicked calls after encountering symptomatic passengers.

“We had customer agents calling the security desk by the hundreds, telling us about individuals that have the symptoms,” the official said. “Our answer was to follow policy,” the official said, which meant they were not to be kept off aircraft unless they were demonstrably unfit to fly or had recently traveled to China.

Those who arrived before the restrictions kicked in faced crowded planes and extended waits even without the additional layer of medical screenings. But the next wave of travelers, which began arriving March 14, confronted scenes out of a public health nightmare.

Teeming crowds

Trump has spent much of his presidency fixated on U.S. borders and denying entry to foreigners. Of the possible responses to a pandemic, imposing travel restrictions is the one move Trump should have mastered.

The travel ban on majority-Muslim countries that Trump declared during the first days of his presidency triggered chaos at airports and border entry points. The fallout delivered an early lesson on the consequences of wielding power without adequate planning.

When Trump moved to block travel from China in January, there were few indications of disruption at affected airports. But while the president has depicted that decision as one he made before anyone else recognized it was necessary, in reality major airlines were forcing his hand.

Delta and American had announced on Jan. 31 they were suspending routes to China before Trump announced the restrictions. United informed the White House it had already decided to do the same but was willing to hold off on announcing it publicly if Trump was prepared to act swiftly in issuing an order, officials said. Eager to claim credit for acting to contain the virus, Trump’s announcement came within hours.

The Europe restrictions followed six weeks later but unleashed chaos in ways that surpassed even that of the Muslim ban.

Current and former officials said key agencies, including the Departments of Homeland Security and Transportation, had no meaningful input in the nature of the Europe restrictions or how and when they would be executed. An administration official said officials from both agencies were present at meetings where the ban was discussed.

The administration scrambled to round up contractors to conduct temperature checks on tens of thousands of passengers. Officials said the magnitude of the mobilization was unprecedented. Even so, the contractors were overrun by the rush of travelers that Trump had helped unleash.

Even the most basic screening steps seemed to backfire. The CDC failed to distribute a new paper questionnaire in time for it to be shared with airlines in advance, meaning passengers had to fill it out upon arrival. As a result, travelers found themselves reaching around one another for slips of paper and pencils, risking transmission as the bottlenecks got worse.

The number of arriving passengers had in fact plummeted by the first day under the new restrictions. Just 19,418 passengers arrived from designated countries in Europe, according to CBP, less than half the number from the previous day. But even the dramatically reduced passenger volume seemed to overwhelm airport screeners.

Alarming photos and expressions of outrage lit up social media throughout March 14. “To find yourself waiting four hours in a crowded customs hall is not social distancing,” a passenger arriving in San Francisco posted. “Fix that or fail.”

A photo showed thousands of travelers in line at Dallas-Fort Worth without masks or other protection. “This will not flatten the curve,” the caption accompanying the tweet said.

Even JFK Airport in New York had “turned into a #CoronaVirus breeding ground,” one traveler tweeted, where teeming crowds were being subjected to “useless enhanced #COVID19 screening measures.”

But the most disturbing scenes emerged from Chicago’s O’Hare. By late evening, the conditions had become so unsafe that Illinois Gov. J.B. Pritzker (D) began delivering broadsides on Twitter.

“The crowds & lines at O’Hare are unacceptable and need to be addressed immediately,” he tweeted at 10:50 p.m. “Since this is the only communication medium you pay attention to,” he said, taking explicit aim at the president, “you need to do something NOW.”

He ended with one final blast: “The federal government needs to get its s@#t together.”

Pritzker’s aides had struggled to get answers from the administration earlier in the day, but the Twitter outburst got the White House’s attention. Within minutes, Douglas Hoelscher, director of the Office of Intergovernmental Affairs, phoned Pritzker. But instead of vowing to fix the problems at O’Hare, Hoelscher began criticizing the governor for insulting the president and said Pritzker should have just contacted the White House.

The conversation grew heated, with the governor saying the White House had failed to communicate or properly implement its plans, according to two people familiar with the exchange.

“There was a lot of yelling,” one of them said.

Others responded in more productive fashion. At 12:30 a.m. on March 15, Chad Wolf, the acting DHS secretary, tweeted that his department was “aware of the long lines for passengers who are undergoing increased medical screening requirements.” He said the department was “working to add additional screening capacity” and pleaded with the public for patience.

The next day, DHS officials identified procedural problems at O’Hare that helped explain why waits and lines there were worse than at other airports. Acting on instructions of supervisors, CBP agents were holding up passengers until all the screening data collected from them had been entered into department computers. Other airports had scrapped the paperwork, putting it off until later, soon after lines began to bulge.

Once O’Hare did so, officials said, the crowds and lines began to dissipate. The critical problems had largely subsided by late Sunday. The lines continued to shrink in the ensuing weeks as Europe travel plunged.

European strains multiplied

Within hours of Trump delivering the Oval address, experts were warning that it was already too late.

Bossert, the former homeland security adviser at the National Security Council, raised fundamental questions about the travel ban in an email he sent public health experts and others late in the evening on March 11.

“Can anyone justify the European travel restriction, scientifically?” Bossert asked the group, which had given itself the moniker Red Dawn in reference to the 1980s movie. “Seriously, is there any benefit?”

The resounding answer he got from others was, “No.” The virus was already too widespread in the United States for travel curbs alone to make any difference. The only chance to contain the outbreak and save lives, some argued, was to impose drastic mitigation measures that would bring social interactions, as well as the economy, to a standstill.

Much of the data that has emerged about the pandemic in the ensuing months appears to validate that view.

Comparing genetic signatures of different strains of the virus has enabled researchers to map its global detonation with growing precision. After surfacing in China in late December, the contagion had migrated to Europe by early February.

There was a fleeting window of perhaps weeks when blocking travel from Europe might have shielded the Eastern Seaboard.

But by mid-February, European strains were established in New York, where they multiplied in the city’s crowded streets and subways before fanning out to the rest of the country, according to findings released by Trevor Bedford, a professor of epidemiology at the University of Washington.

The virus then continued crossing the Atlantic — probably in both directions — for weeks before the Trump administration acted. In February alone, more than 1.8 million air travelers from Europe entered the United States, according to the Bureau of Transportation Statistics. Those travelers wouldn’t have faced even a temperature check.

An April study led by researchers at Northeastern University in Boston concluded that New York probably had more than 10,000 undetected cases by March 1 — two weeks before the Europe restrictions were imposed — with thousands more cases in San Francisco, Chicago and other cities.

“Horse out of the barn,” said Stuart Ray, a professor of medicine at Johns Hopkins University and expert on infectious diseases. The travel restrictions “could have bent the curve downward” only if deployed alongside massive testing, distribution of protective gear on an enormous scale and clear public messaging about social distancing.

“Without those,” he said, “transmission would have overtaken any benefit of travel ban.”

Some in the Trump administration argue that such assessments are too pessimistic. Without the Europe restrictions, “you would have probably seen a higher seeding in the United States,” and infections would still be rising, one official said. “This is the advice we were getting from Birx, Fauci and others.”

But setting aside the issue of timing, key components of the screening measures appear to have failed. Temperature checks, for example, have proved to be an unreliable way to identify carriers of the coronavirus because many of the most infectious individuals are, at least for the moment they face a thermometer, asymptomatic.

The plan also depended on authorities’ ability to trace individuals exposed by incoming travelers. This typically entails obtaining passenger manifests from airlines and contacting anyone who sat within several rows of someone who tests positive. Officials said the CDC has struggled to get information needed for “contact tracing” in a timely manner from airlines.

But that protocol was rendered pointless by the chaotic scenes in airports, and the resulting contacts that would be impossible to trace.

Siebert, the student who studied abroad, appears to have encountered all of these issues upon his return from Madrid. After filling out the CDC questionnaire and reporting his previous symptoms, the screener took his temperature and stepped away briefly.

“You’re good, just go self-isolate,” the screener said when he came back, according to Siebert. Exhausted, the New York University drama student retrieved his bags and was greeted by family members who took him home.

Siebert, 21, said he was never contacted about any of the information he reported to officials at the airport. The next day, he independently went to be tested at Northwestern Memorial Hospital in Chicago. A day later, the results came back confirming his infection.

“Ultimately, I am a culprit in bringing coronavirus back to the United States,” he said. His mother also came down with the illness, though her symptoms appeared before Siebert’s return. The two isolated themselves for weeks in the household, he said, and no other family members became sick.

Siebert was among 110,000 passengers screened during the first four days of the Europe travel restrictions. According to the CDC, only 140 cases of infection were identified either by airport evaluations or subsequent test results reported to the center by local health authorities.

If other travelers were exposed by Siebert’s infection, it is unlikely any of them were ever told. A CDC spokesman said the center has conducted “contact tracing” investigations on nine Europe-to-United States flights since the restrictions began. Iberia Flight 6275 — the one Siebert took to get home — was not among them.

Julie Tate contributed to this report.