But in explaining to Americans why he ordered US strikes in Syria, he made the most conventional of arguments: There are certain acts -- such as the use of chemical weapons against civilians -- that so offend Western values, it is incumbent on the United States to act.

"Today, the nations of Britain, France and the United States of America have marshaled their righteous power against barbarism and brutality," Trump said. "Tonight, I ask all Americans to say a prayer for our noble warriors and our allies as they carry out their missions."

The speech, delivered on Friday night as US strikes rained down on Syria, offered a stirring rationale for a strong American role in the world -- yet it also exemplified the contradictions and inconsistencies of Trump's presidency.

He has often criticized NATO and America's European allies. But here, he was acting in the familiar role for an American President as the moral leader of the West, lining up against the transatlantic alliance's traditional foe, Russia.

Trump's speech also exemplified the competing strands of his "America First" foreign policy.

Just two weeks ago, Trump had said it was time to get troops out of Syria to let regional powers fix the morass there. But on Friday, he promised a sustained military campaign to prevent President Bashar al-Assad using chemical weapons. At the same time, he warned against the illusion that American action could mend a broken region, exemplifying a common, ambivalent thread between his administration and that of Obama, a predecessor who he disdains.

Friday's action in Syria also forced Trump to confront two conflicting sides of his nature -- the desire to look tough as a military leader and his instinct, distilled from endless 2016 campaign rallies in the heartland to avoid Middle Eastern entanglements.

And with US personnel in action in perilous Middle Eastern skies, Trump invoked national unity and the moral authority of the presidency, at a moment when he is being accused of squandering both as legal and political scandals swirl around him.

The speech, delivered against a backdrop of the White House's brilliantly colorful wallpaper in the Diplomatic Reception Room could turn out to be a turning point for a White House that has often seemed to reject the traditional US global role. But equally, it could end up being a brief scripted interlude in an administration defined by chaos and improvisation.

American power

For now, the desire to project American power -- in a way that sends a clear message to autocrats like President Vladimir Putin of Russia and Chinese President Xi Jinping -- whom Trump admires -- won out over the President's antipathy to entanglements in a troubled region.

At the same time, by plunging as never before into the quagmire of Syria, a cockpit of regional rivalries where Russian, Iranian, American and Turkish forces rub shoulders, the President was forced to deal with a complex foreign policy challenge that defies his simple win-loss worldview.

Sources told CNN on Friday that Trump had been pushing for a robust military response to the chemical weapons attacks, but that Defense Secretary James Mattis had warned about the danger of inciting tensions with Russia and Iran, Assad's sponsors who have substantial forces in war-torn Syria.

Exact details of the range of targets and the scale of Friday night's operation were still unclear. But it appears to have been planned to be more formidable than the US cruise missile strikes a year ago to punish a previous chemical weapons attack but not to cross a line that could draw Russia and Iran into a dangerous conflict.

Still, the speech marked another step in the evolution of Trump's position on Russia, from deference and conciliation towards an increasing willingness to directly confront Putin.

"Russia must decide if it will continue down this dark path, or if it will join with civilized nations as a force for stability and peace," Trump said, addressing a nation accused of meddling in the US election to help him win.

'Uniquely dangerous'

JUST WATCHED Trump: US launched strikes on Syria Replay More Videos ... MUST WATCH Trump: US launched strikes on Syria 00:29

The most striking aspect of Trump's address was his defense of principles cemented in the 20th century which unite the transatlantic powers.

The President argued that the air attacks on Syrian targets were justified to reinforce the principle that some weapons are so heinous, they have no place in the arsenals of civilized nations.

"Following the horrors of World War I a century ago, civilized nations joined together to ban chemical warfare," Trump said. "Chemical weapons are uniquely dangerous not only because they inflict gruesome suffering, but because even small amounts can unleash widespread devastation."

Trump was speaking exactly 100 years after increasing numbers of American troops arriving on the Western Front began to turn the course of World War I, after years of carnage that bled France and Britain dry.

So the compositon of the US-led coalition that struck Syria on Friday night was symbolic at a time when Trump's own criticisms of NATO and his anti-EU populism has raised doubts about his commitment to the Western alliance.

It may also renew faith in transatlantic cooperation at a time when totalitarian states like China, Russia and Turkey are challenging the preeminence of liberal democracies in a new great game of geopolitics unfolding across Eurasia.

Still, in the Middle East, the irony of the US-Britain-France axis might strike some as ironic given that the borders drawn across the Arab world by the victors of World War I are at the root of many modern day conflicts.

Trump's commitment to a pan-Atlantic effort to reinforce 20th Century liberal values is a victory for French President Emmanuel Macron, who since his stunning election win last year has emerged as the leader of Europe and has invested considerable political capital in binding Trump into the alliance.

JUST WATCHED Mattis: This is a one-time shot, for now Replay More Videos ... MUST WATCH Mattis: This is a one-time shot, for now 00:48

Despite Trump's invoking of perennial values, his speech also reeked of skepticism of his own actions in committing the United States to a conflict in Syria that has already cost hundreds of thousands of lives.

"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," Trump said.

The President also appeared to hint at the doctrine of his new foreign policy team, that includes new national security adviser John Bolton, who is a hawk but skeptical of the neoconservative instincts of many of his former colleagues in the Bush administration.

"Looking around our very troubled world, Americans have no illusions. We cannot purge the world of evil or act everywhere there is tyranny," Trump said.

"No amount of American blood or treasure can produce lasting peace and security in the Middle East. It's a troubled place. We will try to make it better, but it is a troubled place" he said.

"The United States will be a partner and a friend, but the fate of the region lies in the hands of its own people," Trump said, days after demanding more effort by US allies in the Gulf to stabilize Syria.

Political plight

Though Trump's address did not mention his own tenuous political plight, it was impossible to separate it from the riotous political climate in Washington.

The new US assault in Syria came at a moment when Trump is under siege on multiple legal fronts -- from special counsel Robert Mueller and after the raids targeting his personal attorney Michael Cohen in New York on Monday.

Trump is also being castigated by fired FBI Director James Comey, whose new book paints the President as a relentless liar and someone whose presidency and personal conduct is antithetical to the rule of law and US values.

As he was contemplating the grave decision to send American forces into combat, Trump spent part of Friday afternoon lambasting Comey and his former colleagues on Twitter as "a den of thieves and lowlifes."

The tweet, in the light of his later address to the nation was a reminder of the gap between the scripted and spontaneous versions of Trump.

It remains to be seen if a President who has approval ratings in the 40s and who has made little attempt to bring the nation together can be an effective war leader -- especially if Friday's attacks do not turn out as planned.

Trump's critics are already accusing him of seizing on the situation in Syria to deflect attention from his political plight at home.

That may be true to some extent, yet the serious themes raised in his remarks undercut that impression to some extent.

And Comey's attacks may now strike a discordant note now Trump is a commander in chief leading US troops in battle.

Still, as President Clinton demonstrated after launching cruise missiles against terrorism targets in Afghanistan and Sudan in 1998 at the height of the Monica Lewinsky scandal investigation, "wag the dog" strategies only work for so long.

In the end, the legal process is impossible to shake.