WASHINGTON (BLOOMBERG) - Donald Trump abandoned his instincts as an impulsive and go-it-alone President in Syria, deliberating at length with his military commanders and working closely with US allies in Europe to confront President Bashar al-Assad for an alleged chemical weapons attack.

Before unleashing airstrikes with the UK and France on Friday (April 13) night, Mr Trump spent days conducting an intensive review of options and strategies with Pentagon officials and his National Security Council, according to a White House official.

The President discussed possible targets and asked detailed questions about each option, said the official, who described the deliberations on condition of anonymity.

Participants included Vice-President Mike Pence, Defence Secretary James Mattis and Mr John Bolton, Mr Trump's new national security adviser.

Mr Trump's handling of the American response to the April 7 chemical attack blamed on the Assad regime is emerging as a test of his self-portrayal as a more decisive commander-in-chief than his predecessors.

The coordinated military response also provided the President's actions heightened international legitimacy and protects him politically at home, where a lone military venture in the Middle East might well be unpopular.

Mr Trump has criticised President George W. Bush's administration for the war in Iraq.

"The combined American, British and French response to these atrocities will integrate all instruments of our national power - military, economic and diplomatic," Mr Trump said in nationally televised remarks.

Criticism of the allied air strikes was limited. Vice-President Mike Pence cut short his schedule at a summit in Lima, Peru, to call Republican and Democratic congressional leaders ahead of Mr Trump's announcement, his spokesman Jarrod Agen said.

French President Emmanuel Macron proved helpful in the planning of the latest Syrian campaign, the official said. He and Mr Trump spoke repeatedly this week.

By working with two other allies, and spending several days consulting with military commanders and leaders in Europe and the Middle East, Mr Trump's approach differed from a unilateral US missile attack against Syria almost exactly a year ago in the aftermath of another chemical attack blamed on Mr Assad's forces.

The strategy that unfolded this week included a deception in which navy warships were manoeuvred to persuade the Russians - falsely - that the ships would take part in a strike, the official said. The US has not yet said how the air strikes were actually conducted.

In his nationally televised remarks, Mr Trump took pains to isolate Russian President Vladimir Putin for his support of Mr Assad's regime.

Both Mr Trump, and in a later briefing General Mattis, went to lengths to call on other countries to join the opposition to Mr Assad.

Mr Trump called out Mr Putin for failing to eliminate Syria's chemical weapons as Russia had pledged to do in 2013.

"Russia must decide if it will continue down this dark path, or if it will join with civilised nations as a force for stability and peace," Mr Trump said. "Hopefully, someday we'll get along with Russia, and maybe even Iran - but maybe not."

The White House said in a statement on Friday night that the US had assessed "with confidence" that Mr Assad's forces were responsible for the April 7 chemical weapons attack in Douma, a Damascus suburb.

"A significant body of information points to the regime using chlorine in its bombardment of Douma, while some additional information points to the regime also using the nerve agent sarin," the White House said.

Dozens of people were killed, including women and children, and hundreds more were injured, according to the statement.

Mr Trump, who evoked the devastating poison gas attacks of World War I, said the US was prepared to "sustain this response until the Syrian regime stops its use of prohibited chemical agents".

But he called on regional forces to contribute.

"America does not seek an indefinite presence in Syria under no circumstances. As other nations step up their contributions, we look forward to the day when we can bring our warriors home," Mr Trump added.

Germany, another important European ally, stayed out of the conflict despite US urging, the White House official said.

Chancellor Angela Merkel condemned the use of chemical weapons and cited "very strong evidence" that the Syrian regime had again deployed them, but the country ruled out participating in any military action.

At the same time, she made clear that Germany wouldn't stand in the way of a response by its allies.

Germany should push for a special summit to formulate a joint European Union position on Syria and plans for a peace initiative, Dr Wolfgang Ischinger, chairman of the Munich Security Conference, said on Deutschlandfunk radio on Friday.

Dr Ischinger said the US and Russia are "behaving irresponsibly. It's alarming but it's not a reason to panic."

While the US has not yet released an assessment of its attack, initial reaction from some Syria experts was positive.

The US and allied approach "seems smart and concerted" while representing a "much larger strike than last year", said Mr Andrew Tabler of the Washington Institute for Near East Policy.

Dr Jamil Jaffer, founder of the National Security Institute at George Mason University's Scalia Law School, said that Mr Trump, British Prime Minister Theresa May and Macron were to be "applauded for this strong action".