WASHINGTON (AP) — From the earliest days of his first presidential campaign, President Trump aggressively challenged the pillars of Republican foreign policy that defined the party since World War II.

He mocked Senator McCain’s capture during the Vietnam War, validated autocrats with his platitudes, questioned longtime military and security alliances and embraced an isolationist worldview. And to the horror of many GOP leaders at the time, it worked, resonating with voters who believed, in part, that a bipartisan establishment in Washington had brokered trade deals that hurt American workers and recklessly stumbled into so-called “forever wars.”

But Russia’s brutal invasion of Ukraine is posing a serious test for Mr. Trump and his “America First” doctrine at a moment when he is eyeing another presidential run and using this year’s midterm elections to keep bending the GOP to his will.

He’s largely alone in his sustained praise of President Vladimir Putin as “smart,” an assessment he reiterated last week during speeches to donors and conservative activists. His often deferential running mate, Vice President Pence, split with him on the issue late Friday.

The multinational partnerships that Mr. Trump repeatedly undermined, meanwhile, have allowed the West to quickly band together to hobble Russia’s economy with coordinated sanctions. The NATO alliance, which Mr. Trump once dismissed as “obsolete,” is flexing its strength as a foil to Russia’s aggression.

Perhaps most fundamentally, the war is a fresh reminder, observers say, that America can’t simply ignore the world’s problems, even if that’s sometimes a politically appealing way to connect with voters facing their own daily struggles.

“This is a brutal wake-up call to both parties that not only are we not going to be able to do less in the world,” said the president of the Council on Foreign Relations, Richard Haass. “We are going to have to do more.”

While he argued that large elements of both parties have demonstrated a desire to turn inward, the current situation poses a “special problem” for Republicans and the “America firsters” who have previously tried to paint Russia as a benign actor.

“The entire thrust of America First, I would argue, was misguided in a world where what happens anywhere can and will affect us,” he said.

It’s unclear whether the Western unity that has taken hold against Russia can be sustained if the war escalates, expands beyond Ukraine or drags on indefinitely. And after two decades of American foreign policy failures, including the Iraq War and the botched withdrawal from Afghanistan, many Americans are approaching the moment with caution.

On the eve of Russia’s invasion, just 26 percent of Americans said they supported the United States playing a major role in the conflict, according to a poll from The Associated Press-NORC Center for Public Affairs Research.

But the challenges to Mr. Trump’s approach to the world are clear.

Sweden and Finland have abandoned their long-held neutrality and warmed to the idea of joining NATO, expanding an alliance Mr. Trump continued to criticize this week. Germany, a country Mr. Trump spent years trying to browbeat into spending more on its defense, broke its longstanding post-World War II policy by sending anti-tank weapons and surface-to-air missiles to Ukraine and pledging to dramatically increase its defense budget.

Mr. Trump and his allies insist that Russia would never have invaded Ukraine were he still president. And Russia did not make aggressive moves on his watch, something former aides and others credit to his erratic behavior and direct threats that left world leaders uncertain of how Mr. Trump would respond to a provocation.

The Washington director of the Ronald Reagan Presidential Foundation and Institute, Roger Zakheim, credited Mr. Trump for deterring Mr. Putin, who he said had “validated the need for allies to invest more in their security and defense.”

“I think President Trump, at least as it related to Ukraine, was able to deter Vladimir Putin. And that was a function of unpredictability, which is valuable to deterring an autocrat like Vladimir Putin,” he said. Still, he argued Mr. Putin’s actions had been “so aggressive and so brazen and so immoral” that it had “de-emphasized the difference” between various foreign policy approaches.

Still, the war renews focus on the controversial role Ukraine played during Mr. Trump’s tenure, particularly the way the then-president used defense of the struggling country as a bargaining tool to improve his domestic political standing.

Mr. Trump was impeached for the first time for trying to pressure Ukraine to investigate his 2020 Democratic opponent, President Biden, and his son, Hunter Biden. The effort included holding up nearly $400 million in U.S. security aid to Ukraine and leveraging an Oval Office visit that Ukraine’s president, Volodymyr Zelensky, had been requesting.

Mr. Trump also pushed discredited claims that Ukraine, not Russia, had meddled in the 2016 election, repeatedly siding with Mr. Putin over his own national intelligence agencies.

“Putin is the critical agent, but certainly Trump contributed to it with his scheme back then and continued to contribute it by undermining national security,” said retired U.S. Army lieutenant colonel Alexander Vindman, the former national security council whistleblower who raised alarms about Mr. Trump’s pressure tactics. “Ultimately the president undermined U.S. foreign policy because he weakened Ukraine.”

As he aims to play a significant role in this year’s midterms and potentially run for president again in 2024, Mr. Trump has shown little interest in calibrating his approach to Mr. Putin.

State Secretary Pompeo, who has been laying the groundwork for his own potential presidential run, has largely abandoned the language he was criticized for using before the invasion, when had called Mr. Putin “very capable” and said he had “enormous respect for him.” Even Fox News host Tucker Carlson, who had openly questioned why he shouldn’t side with Russia over Ukraine, has tried to walk back his pro-Russia rhetoric, saying, “We’ve been taken by surprise by the whole thing.”

That’s left Mr. Trump relatively isolated, defending his decision to label Mr. Putin as “smart” and criticizing the response from Mr. Biden and other Western leaders, even as he has denounced the invasion as “horrific” and a “very sad thing for the world.”

“NATO has the money now, but they’re not doing the job they should be doing,” he said this week on Fox Business. “It’s almost like they’re staying away.”

That has earned rebuke from some in his party.

In a speech to GOP donors Friday night, Mr. Pence forcefully defended NATO and admonished those who have defended Mr. Putin as he, too, weighs a presidential run.

“There is no room in this party for apologists for Putin,” he said, according to his prepared remarks. “There is only room for champions of freedom.”

Senator McConnell told Fox News there “should be no confusion about Vladimir Putin.”

“He’s a thug. He’s a killer,” Mr. McConnell said. “He’s been on the rampage and this will not end well for him.”

A senior fellow at the right-leaning American Enterprise Institute think tank and a contributing editor of The Dispatch, Chris Stirewalt, said Russia’s invasion of Ukraine is fundamentally different from the wars in Afghanistan and Iraq that turned large swaths of the American public against foreign intervention and which Mr. Trump was able to use to his political advantage.

“Putin,” he said, “has undone so much of what Trump and nationalists in the United States had done to change the global order.”