During the presidential election campaign, Donald Trump was adamant: The United States should not get involved in more foreign conflicts. Toppling dictators from Saddam Hussein to Moammar Gadhafi had led to disaster, Mr. Trump insisted, and he had no interest in fighting Syrian strongman Bashar al-Assad.

But in the first major military operation of his presidency, Mr. Trump ordered 59 missiles fired at one of Mr. al-Assad's air bases Thursday as retribution for a sarin gas attack that killed more than 80 Syrian civilians, including children. And in a sharp change from Mr. Trump's policy of defending only U.S. interests, the President framed the imperative to act in humanitarian terms.

"Even beautiful babies were cruelly murdered in this very barbaric attack," Mr. Trump said Thursday night as he announced the strike. "No child of God should ever suffer such horror."

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The move appeared politically astute. Despite angering some of Mr. Trump's base, it drew the backing of politicians on both sides of the aisle in Congress and plaudits from leaders around the world.

It also offered a window into the decision-making style of the new President, even if the exact reason for such an abrupt about-face remained unclear.

Mr. al-Assad's brutality was well-known long before this week – in 2013, he gassed thousands of civilians in the Damascus suburbs – but Mr. Trump did not support acting against him, pointed out Rebecca Lissner, an expert in U.S. defence strategy and decision-making. Nor did the President previously show concern for protecting human rights in the Middle East.

"If this is part of some policy calculation, it's not clear what that is," said Ms. Lissner, a fellow at the Council on Foreign Relations. "Which makes the emotional explanation for his decision compelling."

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Mr. Trump has had a difficult start to his presidency, with major policy pushes on immigration and health care thwarted, and his White House riven with infighting. It is possible, Ms. Lissner said, that Mr. Trump saw this as an opportunity to finally take concrete action on a major file. The move was also "politically savvy," she said, pointing to the broad support it has received across the spectrum as evidence it could raise the President's stock with more centrist voters.

Nancy Pelosi, the Democratic leader in the House, approved of the strike as a "proportional response" to Mr. al-Assad's brutality. Senate Majority Leader Mitch McConnell enthused: "America is back." The few voices of dissent were largely those of far-right commentators in pro-Trump forums, who accused the President of adopting the "globalist" policies of erstwhile presidential rival Hillary Clinton.

One of Mr. Trump's biographers, Tim O'Brien, said efforts to find a measured assessment behind the President's thinking are misplaced.

"People will tie themselves in knots trying to discern a linear, rational decision-making [process] from Trump," he said. "It's never been part of his character and it's never going to be."

Gordon Adams, a former national-security official in Bill Clinton's White House, said Mr. Trump's swift decision did not have the hallmarks of the sort of careful process – typically involving staffers from the State and Defence departments, and the intelligence agencies – that would normally be used to help a President decide whether to take military action.

The best explanation for it, he said, is that Mr. Trump wanted to be tough – and show up former president Barack Obama, who famously declared chemical weapons a "red line" in Syria, but then failed to punish Mr. al-Assad for crossing it.

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"This is a President obsessed with toughness. And if you can say nothing else about deciding to use cruise missiles against somebody else's airfield, it sure as heck looks tough: 'I'm not going to be like that guy Obama. I'm going to look tough,' " said Dr. Adams, professor of U.S. foreign policy at American University in Washington.

The problem with such an approach, Dr. Adams said, is that it doesn't appear to be part of any larger plan and it is not clear how Mr. Trump will handle whatever comes now.

"What did we want Assad to do next? What did we hope Russia would do? There is no apparent link to any strategy," he said. "It's the unpredictability of the actions, I think, that most upsets the apple cart. It drives foreign leaders crazy."

Now, Mr. Trump must manage the consequences of his decision. A major risk is escalation by Russia, Mr. al-Assad's main backer, said David Schenker, a former Pentagon official and expert at the Washington Institute for Near East Policy.

The strike "sends a very clear message to Russia and to the Syrians that there is a new administration that is not averse to using force and that will employ military power other than drone operations," Mr. Schenker said.

In a Facebook post, Russian Prime Minister Dmitry Medvedev accused Mr. Trump of breaking his campaign promises and bringing his country to "the verge of a military clash with Russia" to please the "Washington establishment."

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Still, Mr. Schenker added, the strike does not reflect a major shift in policy. "If they wanted to do regime removal, they wouldn't be hitting an airfield," he said. "This is a limited response to a gross violation of international norms."

Nor does Mr. Trump's invocation of human rights in ordering the strike mean he will back down from attempts to keep Syrian refugees out of the United States. Asked Thursday night whether the administration had considered this possibility, national-security adviser H.R. McMaster was blunt.

"No," he said. "That wasn't discussed as any part of the deliberations."