Gregory Korte

USA TODAY

PALM BEACH, Fla. — As he campaigned for president last year, Donald Trump repeatedly criticized President Barack Obama for his "line in the sand" against Syria — the ultimately empty threat to take military action if the regime used chemical weapons against civilians.

Now, faced with his own line-in-the-sand moment, President Trump has ordered a cruise missile attack against a Syrian air base, opening up a new front in the continuous 16-year war in the Middle East.

The U.S. airstrike follows the apparent use of poison gas against civilians in a rebel-held city in northern Syria Tuesday — an attack that killed at least 86 people, including 27 children.

"Tonight, I ordered a targeted military strike on the airfield in Syria from where the chemical attack was launched," Trump said Thursday night. "It is in the vital national security interest of the United States to prevent and deter the spread or use of deadly chemical weapons."

At a news conference with King Abdullah of Jordan on Wednesday, Trump said the apparent chemical attack, "crossed a lot of lines for me."

"When you kill innocent children, innocent babies — babies, little babies — with a chemical gas that is so lethal — people were shocked to hear what gas it was — that crosses many, many lines, beyond a red line," he said.

More on Syria:

Trump: Syrian strike is in 'vital national security interest' of United States

U.S. launches strikes against Syria: Here's what we know

Syria's civil war: A look at how we got to this point

President Trump finds his line-in-the-sand moment

Until this week, Trump has said very little to suggest his course of action. Indeed, a key facet of his military doctrine has been a refusal to telegraph what he's going to do.

"I watched Mosul, where the past administration was saying, we will be attacking in four months. And I said, why are they doing that?" Trump said Wednesday. "I’m not saying I’m doing anything one way or the other, but I’m certainly not going to be telling you."

Trump continued to play his cards close to his vest most of the day Thursday. "I don’t want to say what I’m going to be doing with respect to Syria," he told reporters on board Air Force One, as he flew to his Mar-a-Lago retreat to meet with Chinese President Xi Jinping. As for Assad, he said only, "Something should happen."

That wasn't always Trump's position. Asked during a presidential debate last year whether he agreed with running mate Mike Pence that the U.S. should be prepared to take military action against Syria, Trump put some distance between himself and his future vice president: "He and I haven't spoken, and I disagree."

Instead, confronted with questions about the humanitarian catastrophe in Aleppo, Trump turned the conversation back to the Islamic State — the terrorist group that both Syria and the U.S. are fighting, and which he sees as the far greater evil.

"I think you have to knock out ISIS. Right now, Syria is fighting ISIS. We have people that want to fight both at the same time. But Syria is no longer Syria," he said.

As Democratic rival Hillary Clinton continued to press for greater U.S. intervention in the Syrian civil war, Trump told the Reuters news agency,"You're going to end up in World War III over Syria if we listen to Hillary Clinton."

Trump's ambivalence about confronting the Assad regime dates back even to Obama's own red-line moment. After Assad used chemical weapons in 2013, thereby crossing Obama's own "red line" that would warrant a U.S. response, Trump made his feelings known on Twitter.

"President Obama, do not attack Syria. There is no upside and tremendous downside. Save your "powder" for another (and more important) day!" he tweeted.​

For Trump, that day has come.