Source: CNN

In a policy shift that will make heads spin, the Trump administration has in the space of a few days gone from a hands-off approach to the civil war in Syria to launching of dozens of Tomahawk missiles against a Syrian air base near the city of Homs.

So why the dramatic pivot? And what are the key takeaways from this development that has broken years of US reluctance to strike Bashar al-Assad directly?

Chemical weapons attacks genuinely moved Trump

President Donald Trump doesn't strike us as a sentimentalist, particularly when it comes to hapless citizens in foreign lands. But perhaps seeing the horrific pictures of the innocent victims -- particularly women and children -- he experienced a stunning moment of pathos and clarity about what was happening in Syria on his watch.

If those photos were supplemented with intelligence briefings describing the possible nerve agents used and their effects, the horrors would have been amplified. The Rose Garden news conference with the visiting King of Jordan – one of the few Arab leaders who has actually stepped up to assist on Syria – provided the perfect opportunity for a presidential moment to speak out against evil and pledge to do something about it. The President clearly felt a responsibility for a heinous act that he saw as a challenge to his leadership – and we know how he responds when challenged.

He went out on a limb

That's the only thing that could have created the rhetoric Trump used in the Rose Garden. Trump all but drew his own "red line" over Assad's use of chemical weapons, which he said "crosses many lines."

Along with UN Ambassador Nikki Haley's implicit threat that the United States would take unilateral action if the UN Security Council didn't act, the administration had virtually put itself in a "put up or shut up" situation. Either the President made good on his red line threat, or he'd have ended up looking weak and feckless, like Barack Obama.

Trump can now draw a sharp line between the weak president who wouldn't act and the stronger president who has. Indeed, not to strike after his Rose Garden moment -- especially this early in his presidency -- would have damaged his credibility significantly.

Signaling to North Korea and Iran

There's not a shred of evidence to suggest that Trump's calculations on Syria relate to the other crisis hanging over his head: what to do about North Korea's nuclear weapons.

Still, it's is easy to imagine how Trump would see a strike on Syria as sending a powerful signal to Kim Jong Un that the United States was prepared to get tougher with him. US action in Syria will get the attention of the North Korean leader and his Chinese protectors. And that's precisely what the administration wants.

Still if this strike leads to a situation where the United States is actively intervening in Syria to change the regime, it could make Kim even more determined to cling to his nukes.

On Iran, the President has done a pretty significant about-face from his campaign rhetoric in adhering to the nuclear agreement. And the strike against Assad renews his pledge to be tougher with Iran and signals to Assad's chief backer that the United States is not a paper tiger and that there may be costs to supporting the Syrian dictator.

The administration is ramping up challenges to Iran's proxies in the civil war in Yemen, but Yemen is not a high strategic priority for the Iranians.

Whacking Assad will get the attention of the mullahs and the Revolutionary Guard. But it will also set the stage for a possible proxy war with the United States in Syria -- a confrontation in which Washington has few reliable allies on the ground.

And Putin makes three

How Putin will play the US strike over time isn't yet clear. The Russians have condemned the attack and threatened both to go to the United Nations and end communications with the United States over air campaigns in Syria. We will find out more when Secretary of State Rex Tillerson goes to Moscow next week.

Putin has no desire to tangle with the United States in Syria, but much of his reaction will be shaped by what the administration intends to do next.

If Washington's action is designed to restrain Assad's use of chemical weapons, the Russians -- who clearly knew about the attack in advance as their personnel were at the base from which it was purportedly launched -- may be embarrassed but willing over time to cooperate in helping to end their use.

But if the strike is designed to set the stage for further US pressure on Syria and indirectly on Russia, Putin will like push back against the US challenge in Syria. And that might set the stage for a US-Russian proxy struggle in Syria, where Moscow has many advantages on the ground.

A one-off or prelude to more?

And that brings us to the key question: What was the purpose of the strike?

Was it an effort to change Assad's behavior on using chemical weapons, or the beginning of a more fundamental US involvement in Syria to change his regime?

Will the United States now strike Assad when he uses conventional barrel bombs against civilians? Is the Trump administration going to ramp up training of the Syrian opposition? And is using military and political power going to begin a serious process of actually ending Syria's civil war?

Clearly, getting rid of Assad will help destroy ISIS, though that depends on whether the Trump administration is prepared to stay the course and is ready with a strategy for the day or more likely the decade after.

And that strategy will require a major investment of resources over a prolonged period of time for which the public, Congress and perhaps the administration may not be prepared.

We have long had our doubts about pursuing such a strategy in Syria, particularly without the region itself -- and especially Sunni Arabs -- playing the lead role. And we've long argued that such an approach may drag the United States into the muck of the Syrian civil war.

But no matter now. In view of this week's US strike -- even though it was necessary and proportional -- there's a reasonable chance that's where we're headed.

This article was originally published by CNN.