As a viral pandemic has spread across continents with a severity and deadliness that world leaders say demands a global response, President Trump has increasingly turned inward.

He has imposed travel restrictions on one-fourth of the world’s population while criticizing other nations’ response efforts, refused to meet with House Speaker Nancy Pelosi, attacked his handpicked Federal Reserve chairman and defied the warnings of his own public health experts.

His handling of the crisis, including a surprise decision to restrict travel from Europe, is drawing criticism at home and abroad, with leaders warning that Trump’s go-it-alone approach is doing harm to an already fraught situation.

German Foreign Minister Heiko Maas said that reasons “less than factual” appeared to have played a role in Trump’s decision-making.

“We can’t adequately meet this challenge — not even within the USA — when decisions being made are garnished with blame,” Maas said in an emailed statement.

Before announcing the travel restrictions in an Oval Office address to the nation Wednesday, Trump compared the death rate in the United States positively with other places where the coronavirus has been spreading, bluntly criticizing U.S. allies as they face a debilitating crisis.

“The European Union failed to take the same precautions and restrict travel from China and other hot spots,” he said. “As a result, a large number of new clusters in the United States were seeded by travelers from Europe.”

On Thursday, he defended his decision to go forward with the 30-day travel ban without first consulting the 26 affected countries. Doing so would take too long, he said, before bringing up an unrelated gripe over trade.

“We had to move quickly,” he said. “I mean, when they raise taxes on us, they don’t consult us.”

Trump’s approach reflects his us-against-them mind-set, tendency to assign blame and combative view of geopolitics — characteristics that have only been amplified by the global crisis, said Russell Riley, a presidential historian at the University of Virginia’s Miller Center.

“It is a persistent, enduring and probably irreversible feature of this president that he’s going to approach a problem on his own and will rise or fall based on his ability to succeed in doing that,” he said, describing Trump’s strategy as a “recipe for a disastrous exercise in leadership.”

The White House defended the president’s leadership, arguing that Trump has been engaging with world leaders about the coronavirus for several weeks.

“As the President said last night, we are using the full power of the federal government and the private sector to protect the American people,” White House spokesman Judd Deere said in a statement. “It is because of the President’s leadership and relationships that he has brought together government and private industry for unprecedented collaboration to curb the spread of the Coronavirus, expand testing capacities, and expedite vaccine development.”

Trump, who called the coronavirus a “foreign virus” and repeatedly said it was “not our fault,” has focused more on its origins than its rapid spread within the country as Americans transfer it to one another. The outbreak has advanced within the country’s borders amid a lack of adequate testing, disjointed messaging and political brinkmanship that even Trump administration officials acknowledge have hampered the public health response.

Trump’s isolationism has extended to the domestic political sphere, where the president has refused to speak with Pelosi (D-Calif.) amid his anger over his impeachment, despite her leading role in determining what kind of emergency stimulus package Congress will produce.

Trump skipped the annual bipartisan St. Patrick’s Day luncheon hosted by Pelosi on Thursday, becoming the first president to miss the event since George W. Bush shortly before the Iraq War began in 2003.

“No, I won’t be going,” he said Thursday. “I have other things to do. I’m very busy.”

Trump has dispatched Treasury Secretary Steven Mnuchin to work with Pelosi on a major relief bill, but a bipartisan agreement remained elusive Thursday.

European leaders — accustomed to Trump’s snubs — felt particular stung by his surprise travel ban targeting more than 400 million people in the 26-nation euro zone.

“The Coronavirus is a global crisis, not limited to any continent and it requires cooperation rather than unilateral action,” European Commission President Ursula von der Leyen and European Council President Charles Michel said Thursday in a joint statement. “The European Union disapproves of the fact that the U.S. decision to impose a travel ban was taken unilaterally and without consultation.”

An E.U. official said the White House has not explained how it decided which European nations to target.

“We don’t know, and we would like to know,” said the official, who spoke on the condition of anonymity to discuss internal deliberations.

Trump has tangled with the E.U. — among Washington’s closest allies — for years on issues of defense spending, climate change and trade. But the travel ban was seen by many European leaders as a reckless move in a time of crisis.

“Viruses do not recognize borders,” former Finland prime minister Alexander Stubb said on Twitter. “Decisions should be based on facts, not politics.”

Trump’s habit of making extemporaneous comments about the coronavirus has isolated him from members of his administration tasked with managing the outbreak.

While the president defended his decision to ban travel from Europe, he brushed off concerns about his administration’s struggles to ensure that tests are available for Americans who have coronavirus symptoms.

“The testing has been going very smooth,” he said Thursday afternoon in the Oval Office. “If you go to the right agency, if you go to the right area, you get the test.”

Around the same time, the nation’s leading expert on infectious diseases was delivering a very different message on Capitol Hill.

“The system is not geared toward what we need right now, what you are asking for,” said Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases and a member of the president’s coronavirus task force. “That is a failing. Let’s admit it.”

There were more than 1,600 confirmed coronavirus cases in the United States as of Thursday evening, and state and local officials said the actual total would be significantly higher if more people were able to get tested.

Since Trump first campaigned for office in 2015, there have been few of the country’s ills he has not tried to link to a foreign source — whether nation-states or immigrants.

He has blamed the U.S. drug-addiction crisis on foreign smugglers and said shuttered factories were exclusively the result of bad trade deals with other countries.

And as recently as this week, Trump cited the virus as added reason to build a physical wall across the southern U.S. border.

Former Trump homeland security adviser Thomas P. Bossert said the president’s focus on immigration control made little sense in the face of a virus that had already begun spreading in communities throughout the country.

“In two weeks, we will regret wasting time and energy on travel restrictions and wish we focused more on hospital preparation and large scale community mitigation,” Bossert tweeted.

Trump said he was surprised by the speed with which the virus has spread around the globe, a product of the increasingly globalized nature of commerce that the president has railed against.

“It’s pretty amazing when you think of what happened and how fast this spread to the world,” Trump said Thursday. “It was one country, then it was four countries, then it was nine. I'm reading this list every week. Then it was 13, then it was 22. And now, I guess, it’s over 100 countries. . . . This is a very fast spreader.”

The virus has taken hold in the United States despite Trump’s unprecedented efforts to use his executive authority to ban large swaths of the global population from being able to travel to the country.

Trump has repeatedly praised his Jan. 31 ban on most travel from China as evidence of forward-thinking leadership.

Fauci and other public health officials have credited Trump for his early decision to restrict travel from China, a country of 1.4 billion, saying the move bought the United States time to prepare.

As he has quizzed his aides about the number of coronavirus cases in the United States, he has also asked how the quantity compares with other countries.

Trump’s partial ban of European travelers, which does not include the United Kingdom, may have some effect on slowing infection in the United States. But since the virus is already spreading rapidly within the country, the ban’s chief purpose may be symbolic and political, said Heather Conley, who directs the Europe program at the Center for Strategic and International Studies.

“It was a domestic political desire to offer a big announcement and to blame someone,” she said Thursday. “And the blame was assigned, last evening, to Europe.”

The ban covers some European countries, including hard-hit Italy, but not others. It also distinguishes between American citizens returning from Europe and noncitizens attempting to travel to the United States.

From a public health perspective, travel bans have limited effect once a virus becomes so widespread, said Harvard University epidemiologist Marc Lipsitch.

“The travel ban seems an unnecessary assault on the economy, but more concerning is that it is the centerpiece of a policy that has inadequately addressed the urgent need to interrupt local transmission in the U.S.,” he said, highlighting the lack of testing by public health labs.

Trump’s decision to ban travel from some European countries but not others opened him up to political attacks from Democrats seeking to oust him from office in the November elections.

“Travel restrictions based on favoritism and politics — rather than risk — will be counterproductive,” former vice president Joe Biden said in a speech Thursday. “This disease could impact every nation and any person on the planet. And we need a plan about how we are going to aggressively manage it here at home.”

James McAuley in Paris and Michael Birnbaum in Brussels contributed to this report.