After the Syrian gas attack, President Trump had a choice: do nothing or do something.

It’s a choice all presidents face, because sooner or later they will be tested—usually sooner, and not just once but many times and in many ways in many places.

When faced with similar circumstances in Syria, Obama declared the existence of a red line and then ignored it. This is one of the worst response/nonresponses possible. It indicates indecision and lack of resolve, a president who talks tough but his threats means nothing and can be safely ignored.

Trump’s reaction was, if not the opposite, something like the opposite. After sending out some signals that he was going to be less interventionist about Assad’s regime in general, he acted more boldly when challenged. His actions—speaking louder than words—would be par for the course for most presidents in a similar situation. They only seem unusual at this point because we’ve grown accustomed to Obama’s post-bluster inaction.

Assad may have calculated that nothing whatsoever would happen, not just because of recent comments by the Trump administration but because of recent history:

Dr. Monzer Khalil, Idlib Province’s health director, said such extreme tactics aimed to demonstrate the government’s impunity and to demoralize its foes.

“It makes us feel that we are defeated,” said Dr. Khalil, whose gums bled after he was exposed to scores of chemical victims on Tuesday. “The international community will stay gazing at what’s happening — and observing the explosive barrels falling and rockets bombing the civilians and the hospitals and the civil defense and killing children and medical staff — without doing anything.”

“Militarily, there is no need,” said Bente Scheller, the Middle East director of the Berlin-based Heinrich Böll Foundation. “But it spreads the message: You are at our mercy. Don’t ask for international law. You see, it doesn’t protect even a child.”…

By showing it puts no limits on the tactics it uses, Mr. Yazigi [an opposition Syrian economist] wrote, “the regime shows to the world the West’s impotence and weakness.”

“The West’s impotence and weakness” started even before Obama. Most of us do not like being the world’s policeman and would prefer not to be (Trump has certainly made many statements to that effect). But if not us, then who? And if no one is the world’s policeman, what then? What message does that send?

That’s the dilemma in a nutshell. It’s the same dilemma that led to the rise of the foreign-interventionist advocates who have been so reviled in recent years, and their fall. But the dilemma remains.

Liberal Democrats used to be more inclined to applaud such interventions, as well. But in recent years many of them seem to have adopted a simple way of reacting that goes like this: anything a Democrat does is good whether it be isolationist or interventionist, and anything a Republican does is bad.

Nor is it the case that Trump’s action in bombing the Syrian air base will heavily undermine Assad’s power. That would take a lot more—something like the Iraq War, which is almost certainly not in the cards, and which would almost inevitably lead to the need to keep a constant presence in the area, a price we are not willing to pay.

Syria may indeed be an insoluble mess. But Trump’s action does signal to Syria—and the world—that a new policeman is in town, and that now there are indeed limits and red lines and costs to crossing them.