Washington: If there was a selfish, inward-looking core to Donald Trump's election-winning "America First" philosophy, it took a back seat to human decency on Thursday, when the President ordered a swift and stunning missile assault on a Syrian regime air base as punishment for a poisonous gas attack that killed more than 80 civilians this week.

But what happens next?  Russia and Iran have condemned the strike. Russian President Vladimir Putin believes the strikes broke international law and seriously damaged US-Russia relations, while Iran reportedly denounced it as "destructive and dangerous".

What will be the impact on the Syrian civil war and the global, US-led war on Islamic State in Syria and Iraq? How might the 60-plus countries in the Trump-led coalition fighting IS respond? How safe are the hundreds of American troops on the ground in Syria?

Putin's spokesman Dmitry Peskov told Russian state media organisation RT that Putin "regards the strikes as aggression against a sovereign nation" and believes the strikes were carried out "under an invented pretext."

A complex conflict just became more complex – Russian aircraft are in the air, and on the ground Russian and Iranian forces prop up Assad's struggling forces as they battle myriad rebels; after two years, and the coalition waging war on IS is making progress, even as half-a-dozen or more neighbouring countries act within the conflict to promote self-serving agendas.

And through all that, millions have been displaced and an estimate half-million civilians have died. Add those, the 80-odd more freshly poisoned in this week's chemical strike in Idlib province.

In Trump taking the American fight to the regime for the first time, there's an inevitable risk of confrontation with Moscow and Tehran – either verbally between capitals, or between their forces on the ground and their aircraft overhead.

US Secretary of State Rex Tillerson spoke by phone with his Russian counterpart Sergei Lavrov about the chemical attack on Wednesday. But instead of Tillerson calling back to inform Lavrov of Thursday's missile strike, the heads-up was left to the Pentagon which informed senior Russian military officers through the 'deconfliction' channel, a Pentagon to Kremlin line intended to ensure the safety of US and Russian military operations.

On Thursday evening, Tillerson pointedly let it be known that Russian President Vladimir Putin had not been personally advised – and that Trump, who was hosting Chinese leader Xi Jinping in Florida, had not troubled himself to call Putin after the strike.

There was always a possibility that Moscow would have alerted Assad to the incoming American barrage. An American official told reporters: "With a lot of Tomahawks flying, we didn't want to hit any Russian planes".

There is a Russian military area within the Syrian-run base that was hit, but the official claimed that no Russian aircraft were on the ground and that the Russian sector in the base had not been targeted.

Compared with 2013, when President Barack Obama failed to act on his "red-line" to the Damascus regime that Washington would launch military strikes against the regime if it used chemical weapons – but didn't, the biggest difference in the Syrian theatre is the hair-trigger complexity introduced by the presence of Russian ground forces the installation of powerful Russian air defence systems that could take down US aircraft.

In two years of American air strikes in Syria, the US aircraft have not come under attack from the Syrian and Russian air defence systems, because the US has mostly been targeting IS while the regime and its sponsors have focused their war on anti-regime forces.

"Both the Syrians and Russians can act as a spoiler," Andrew Exum, who served under Obama as a defence official, told The Post. "American and coalition aircraft have flown around and through their air defence systems for the last two years. [But] If you launch a strike against the regime, it would have every excuse to start lighting up coalition planes with antiaircraft systems".

If US and Syrian or Russian aircraft were to get into a dogfight or were shot down, the Syrian civil war and Trump's priority, the war on IS, would become messier.

Retired General John Allen, who ran the US-led war effort against IS in Syria and Iraq in the Obama years, was ambivalent on the strategic usefulness of Thursday's US missile attack, telling The Washington Post that it might have been decisive in 2013.

Acknowledging his personal devastation in response to Obama's refusal to strike, he said: "It is much harder now – the US has to ask itself a question: How angry do we want to be on this issue? Are we enraged enough morally that we are ready to take action even with the possibility of dead Russians?"

As it is, Thursday's strike has posed a question over the meaning of another Trump priority – his America First policy, under which the horror of the Idlib chemical attack might have been ignored on the grounds that it was not a threat to American interests.

In recent days Assad was not an American problem – Trump's UN ambassador Nikki Haley's declared that "our priority is no longer to sit there and focus on getting Assad out;" and Tillerson, cold-heartedly, insisted that it was up to the war-ravaged Syrians, not the US, to decide Assad's fate.

And even as he condemned the chemical attack, White House spokesman Sean Spicer insisted that the US was unlikely to change its thinking on Assad because of Syrian "political realities".

All that was in keeping with Trump's campaign rhetoric when he mocked previous administrations' campaigns in the Middle East, insisting instead that he put "America first". And as refugee waves poured out of Syria, he insisted that they'd not be admitted to the US.

And earlier, in 2013, Trump mocked the notion of US retaliating against the Syrian regime for its use of chemical weapons. Urging Obama not to get into the Syrian civil war, he famously tweeted: "Again, to our very foolish leader, do not attack Syria - if you do many very bad things will happen & from that fight the U.S. gets nothing!"

Two days later, he tweeted: "President Obama, do not attack Syria. There is no upside and tremendous downside. Save your "powder" for another (and more important) day!"

All that changed overnight. While Tillerson was speaking about a plan to dislodge Assad, Trump's commentary shifted to the point that questions were being asked about how he might channel his rage and revulsion.

We know now.