President Trump on Friday signs an executive order closing U.S. borders to all refugees temporarily and additionally suspending the entry of anyone from Iraq, Syria and five other predominantly Muslim countries. (Susan Walsh/AP)

In just his first week in the White House, President Trump has sought to redefine America’s most lethal enemy in terms far broader than his post-9/11 predecessors.

The net result of Trump’s new approach — outlined in speeches, interviews and executive orders — is a vast departure for a country that has often struggled over the past 15 years to say whether it is at war and precisely who it is fighting.

With a few sweeping moves, Trump has answered those questions with a clarity that is refreshing to his supporters and alarming to some U.S. counterterrorism officials as well as most of the Muslim world.

For Trump and his senior policy advisers, America is locked in a world war for its very survival, and the enemies in this wide-ranging battle are not only radical Islamist terrorists but a chaotic, violent and angry Muslim world.

“The world is as angry as it gets,” Trump said last week from the White House. “Take a look at what’s happening with Aleppo. Take a look at what’s happening in Mosul. Take a look at what’s going on in the Middle East. . . . The world is a mess.”

One day later, in an appearance at the Pentagon and in signing an executive order — “Protecting the Nation From Foreign Terrorist Entry Into the United States” — Trump laid out his plan to deal with what he had described as a vast and pressing threat. He closed America’s borders to all refugees temporarily and additionally suspended the entry of anyone from Iraq, Syria and five other predominantly Muslim countries.

“The optic of this is really awful,” said Nada Bakos, a former CIA analyst, of the refu­gee ban. “What they’ve done goes too far. All it does is help [Islamic State] recruiting.”

Trump also vowed new “extreme vetting measures” to permanently keep radical Islamist terrorists out of the United States and promised to give Christians from the Middle East and other minority religions in the region priority over Muslim refugees.

Finally, he promised to pump new money into America’s military, what he called “a great rebuilding of the armed services of the United States.”

Both former presidents George W. Bush and Barack Obama had defined the enemy in significantly narrower terms while in office, eager to avoid any moves that might make it appear as if the United States was at war with Islam.

For Bush, the enemy was al-Qaeda and state sponsors of terrorism to include former Iraqi leader Saddam Hussein, Iran and the Taliban. Obama insisted that Bush’s definition was a recipe for “endless war” and singled out an even smaller group. To him, the enemy was a series of terrorist death cults that he said were perverting the peaceful religion of Islam.

The executive order on immigration and refugees was produced at a “frenetic pace” that included none of the interagency reviews that characterized similar orders in the Bush and Obama administrations, a senior U.S. counterterrorism official said.

“The process was remarkable,” said the official, who spoke on the condition of anonymity to discuss sensitive internal deliberations. “Nobody in the counterterrorism community pushed for this. None of us ever asked for it.”

Trump described the order as a key cog in an effort to prevent terrorists from entering the United States, but the policy does not affect countries such as Saudi Arabia, Pakistan or Egypt, whose citizens have launched terrorist attacks inside the United States. Not one of the 19 hijackers who struck on 9/11 came from a country targeted by the order.

The measure drew negative responses across the world, some of which was heard by U.S. forces on the ground in the Middle East.

U.S. commanders advising Iraqi forces reported back that their partners were mystified by the order. “It’s already flowing back,” said the senior counterterrorism official. “They are asking, ‘What do you think of us? Do you see us as the threat?’ ”

Some Iraqi lawmakers proposed banning U.S. troops and civilians from entering Iraq — an action, if followed through, that could lead the authorities in Baghdad to turn to Russia and seek more support from Iran.

Iranian Foreign Minister Mohammad Javad Zarif tweeted that the ban would be “recorded in history as a great gift to extremists and their supporters.”

Trump on Saturday described the move as sensible and not aimed at any particular religious group.

“It’s not a Muslim ban, but we were totally prepared,” he told reporters in the Oval Office. “It’s working out very nicely, you see it at the airports, you see it all over . . . and we’re going to have a very, very strict ban and we’re going to have extreme vetting, which we should have had in this country for many years.”

The stark departure from American policy over the past 15 years is a reflection not only of Trump but the somewhat dystopian vision of his closest advisers.

“We’re at the very beginning stages of a very brutal and bloody conflict,” said Stephen K. Bannon, Trump’s chief strategist, in a 2014 speech to a Vatican conference. “We are in an outright war against jihadist Islamic fascism and this war is . . . metastasizing far quicker than governments can handle it.”

Michael Flynn, Trump’s national security adviser, similarly describes the fight against al-Qaeda and the Islamic State as a “world war.”

“We could lose,” he wrote in his recent book, “The Field of Fight.” “In fact, right now we’re losing.”

Those sorts of analyses represent a radical departure from Obama, who believed that the United States had succumbed to a “season of fear” following the 9/11 attacks that produced a disastrous war in Iraq and a betrayal of America’s core values. As commander in chief, he banned torture — a policy Trump has suggested he might revisit — and sought unsuccessfully to close the U.S. detention facility at Guantanamo Bay, Cuba.

“Shameful” was the word that Obama used to describe calls from Trump and other presidential candidates to impose religious tests on refugees or immigrants.

Obama was convinced that groups like al-Qaeda and the Islamic State did not pose an existential threat to the country. Rather, he suggested that the biggest threat came from an overreaction to the attacks that would cause the United States to turn away from the world.

His approach stressed America’s fearlessness in the face of attacks. “That’s who the American people are — determined and not to be messed with,” Obama said in describing his counterterrorism strategy in 2013. “Now we need a strategy and a politics that reflects this resilient spirit.”

Trump, meanwhile, has chosen a different route.