Early Friday morning Middle East Time, Donald Trump enforced the "red line" President Barack Obama drew in August 2012. The Assad regime used chemical weapons. The United States retaliated by dispatching 59 Tomahawk cruise missiles to inflict perhaps disabling damage on a Syrian air base.

The president's decision seems to have resulted, at least in part, from his emotional response to that picture of a Syrian man holding the bodies of his two children killed by gas attacks.

"Assad choked out the lives of helpless men, woman and children," President Trump said in his announcement to the nation late Thursday night Eastern Time. "It was a slow and brutal death for so many. Even beautiful babies were cruelly murdered in this very barbaric attack. No child of God should ever suffer such horror."

Human beings are hard-wired to be horrified by the sight of dead children. The photo of a dead child on a Turkish beach evidently prompted Angela Merkel to welcome 800,000 "refugees" to Germany. The photo of the man and his two dead children may have had the same effect on Trump. The question is whether his action will have the dreadful consequences Merkel's has had.

Republicans, as well as Democrats in Congress, fear it might. It could lead to military intervention, perhaps to quagmire, in Syria, some fear. They do not unreasonably suggest Trump should seek congressional action for more intensive military action there.

But in the meantime, Trump's decision clearly has done one thing: It has established that those crossing a red line invite American military response. That's a lesson that may not be lost on Xi Jinping, who was dining at Mar-a-Lago when the cruise missiles hit, and who was scheduled to hear the next day Trump's demands that China rein in its ally North Korea.

Secretary of State Rex Tillerson had already carefully refused to rule out any action against Kim Jong-un's missile- and nuclear-brandishing regime. Xi now knows that Trump, unlike Obama, is capable of ordering a military attack, and is inclined to do so in what he considers the appropriate circumstances.

Trump has effectively restored the American power that Obama relinquished when he carelessly drew his red line and then, 13 months later, failed to act on it.

I use the term carelessly advisedly. In a Washington Examiner column almost exactly two years ago, I looked back on how the "red line" was established, relying heavily (and of course with attribution) on an excellent May 2013 article by Peter Baker and three colleagues in the New York Times.

The story harks back to August 2012, one week before the opening of the Republican National Convention, when Obama appeared, unusually, in the White House press briefing room and responded to questions. One was on Syria.

Obama's answer, that use of chemical weapons would "change my calculus," was "unscripted," according to an adviser quoted by Baker et al. It was a "surprise" to "some of the advisers who had attended the weekend meetings and wondered where the 'red line' came from." It may have been prompted by Republican candidate Mitt Romney's promise three months before to send troops to Syria to stop the spread of chemical weapons or in anticipation of Romney's proposal, advanced four days after the impromptu press conference, to do so.

To put it another way, Obama may have uttered the fateful words with the lack of preparation and discipline and the focus on skewering political foes which many in the press portray as characteristic of Donald Trump.

In any case, Obama's refusal in March and April 2013 to act in response to intelligence reports of the use of chemical weapons reduced if it did not eliminate the credibility of the United States. So did his wan and utterly unpersuasive insistence that "I didn't set a red line. The world set a red line."

Trump's action is at odds with his campaign rhetoric suggesting he had no appetite for military action in the Middle East. Just as Obama indicated he would take military action and then refused to do so, Trump suggested he wouldn't and did. Those who charge both or either with inconsistency have a point.

But Trump's action restored American credibility, not only in Syria but around the world, just as Obama's inaction undermined it up through last Jan. 20. That credibility could be squandered, or enhanced, by what happens next. But no one's laughing at America's red lines anymore.