But while the decision to strike Syrian government targets marked a sharp reversal in the President's stance on Syria -- one that called for a laser-like focus on ISIS and largely ignoring Assad -- the sharp, 180-degree turn didn't happen over a matter of days.

Months earlier, during an off-the-record holiday gathering with reporters at his opulent Mar-a-Lago estate a week before Christmas, Trump spoke at length about the carnage of the Syrian civil war, revealing that the issue was weighing on him as he prepared to take office. Trump spokeswoman Hope Hicks authorized CNN on Friday to report the contents of his remarks on the topic at the gathering, which CNN attended.

He described the slaughter of civilians in Syria as a "holocaust," and remarked on the "high pain threshold" of the population there.

Trump also described in detail a video he had seen of an elderly woman being shot multiple times in Syria, struggling as she tried to continue to walk.

And then, he acknowledged that the US had a "responsibility" over the devastating Syrian conflict -- the same word he would use months later before approving the launch of 59 Tomahawk cruise missiles to target a Syrian Air Force base.

"I now have responsibility. And I will have that responsibility and carry it very proudly, I will tell you that. It is now my responsibility," Trump said Wednesday during a White House news conference one day before approving the airstrike.

Trump said that the chemical weapons attack "crossed a lot of lines for me," a telling comment that signaled his shifting willingness to strike the Syrian regime.

But even as he spoke broadly during the December cocktail gathering about a US responsibility over the horrors taking place in Syria, he was clearly undergoing a shift in his approach to the conflict there as his moral responsibility as the US President and "leader of the free world" began to sink in.

It was clear Trump was giving more thought to the Syrian conflict that he would soon inherit as president, and he indicated he would be weighing different ways the US could act to stop the atrocities that continued to unfold in Syria.

He declined to say if he would take military action, parroting his oft-repeated line that the US should act first and talk later, rather than telegraph military actions in advance.

Trump spoke at length about the "tragedy" unfolding in Syria and the atrocities the world had grown all too accustomed to seeing and his demeanor and thoughtfulness as he addressed the topic struck reporters who had covered Trump's campaign since its early days as a crucial piece of insight into how Trump was becoming more clearly aware of the real world implications of his startling election victory.

With his strikes against a Syrian government target on Thursday night, Trump made clear that his Middle East policy won't just be driven by a single-minded focus on terrorist groups like ISIS or core national security threats -- but could be expanded to address the United States' moral responsibility in the world.

That's not something Trump spoke about during his campaign for president. Rather, Trump touted an "America First" philosophy that he said would drive his domestic and foreign policy. He decried the Iraq War as a "stupid" decision that had led to needlessly spilled blood and treasure in the Middle East.

Trump called for keeping US military action in the Middle East singularly focused on eliminating ISIS and argued that the US had "bigger problems" than Syrian President Bashar al-Assad.

"We cannot be the world's policeman," Trump said at his first presidential debate against former secretary of state Hillary Clinton, whom Trump called out for her role in some of the very foreign interventions Trump railed against on the stump.

Trump's internal shift didn't translate into US policy until this week after the latest images of the Assad regime's horrific violence against civilians were broadcast around the world.

Secretary of State Rex Tillerson insisted that the decision to strike was not triggered by an "emotional reaction," but made clear the decision was driven by Trump's refusal to "turn a blind eye" to the Syrian violence.

The chemical weapons attacks that prompted Trump to strike had happened "on his watch," Tillerson said of Trump.

As recently as last week, the administration's top officials continued to articulate Trump's realpolitik view of the Syrian civil war as he articulated it on the campaign.

"With respect to Assad, there is a political reality that we have to accept," White House press secretary Sean Spicer said last week. "The United States has profound priorities in Syria and Iraq, and we've made it clear that counterterrorism -- particularly the defeat of ISIS -- is foremost among those priorities."

Thursday night's strike sent a clear message that Trump was willing to act beyond those counterterrorism priorities, but US officials made clear that the strike was meant to send a message -- not fundamentally alter US policy in the region.

"I would not in any way attempt to extrapolate that to a change in our policy or posture relative to our military activities in Syria today. There has been no change in that status," Tillerson told reporters after the cruise missiles had struck their targets.

Lt. Gen. H.R. McMaster, the president's national security adviser, said the strike "should" shift Assad's calculus on chemical weapons.

Still, he acknowledged that the strike did not eliminate Assad's capacity to conduct the ghastly attacks.

CNN's Noah Gray contributed to this report.

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Trump's evolution on Syria didn't happen overnight - CNN