WASHINGTON — President Trump warned on Wednesday that he would not tolerate the “heinous” chemical weapons attack in Syria, opening the door to a greater American role in protecting the population in a vicious civil war that he has always said the United States should avoid.
The president declined to offer any details about potential action. But he said his horror at the images of “innocent children, innocent babies” choked by poison gas in a rebel-held area of Syria had caused him to reassess his approach. Only days after the White House declared it would be “silly” to persist in trying to oust President Bashar al-Assad of Syria, Mr. Trump said, “My attitude toward Syria and Assad has changed very much.”
“It crossed a lot of lines for me,” the president declared at a news conference in the Rose Garden, referring to the “red line” that his predecessor, President Barack Obama, had drawn before a 2013 poison-gas attack by Mr. Assad’s forces. Mr. Obama’s failure to strike Syria after that, Mr. Trump claimed, sowed the conditions for this new assault. The estimated death toll was reported to have exceeded 100.
Syria was one of several places, along with North Korea and Iran, where Mr. Trump on Wednesday threatened a forceful American response. But in all these cases, he declined to disclose options, arguing that there was a need for surprise but stoking worries that his fledgling administration is not ready to deal with multiple threats across the Middle East and Asia.
At the United Nations, Mr. Trump’s ambassador, Nikki R. Haley, warned that the United States might take unilateral action if the Security Council failed to respond to this latest atrocity in Syria. A shift in policy could include airstrikes, which were considered and ultimately rejected by Mr. Obama.
The president, standing alongside King Abdullah II of Jordan at the news conference, told reporters, “I’m not saying I’m doing anything one way or the other, but I’m certainly not going to be telling you.”
Mr. Trump’s stern words and lack of specifics attested to a leader, 75 days into his presidency, who is determined to show a more muscular style than Mr. Obama but is grappling with many of the same complexities that dogged his predecessor. And they raised anew a question that Mr. Trump until now has avoided: his criteria for using force, both in a humanitarian cause and in facing a direct, if distant, threat to the United States.
“It is usually better to threaten unspecific consequences until you are at a more advanced stage of planning,” said Walter Russell Mead, a foreign policy expert at Bard College. “The danger is you become so distracted by these multiple crises that you can’t focus on the most urgent one, or the one where the U.S. actually has a chance of succeeding.”
Mr. Trump’s challenge is complicated by the new upheaval in the ranks of his national security aides, with the abrupt removal of his chief strategist, Stephen K. Bannon, from the senior policy panel of the National Security Council. On Thursday, Mr. Trump is to meet President Xi Jinping of China in Florida, where the president plans to push for more Chinese support in the campaign to pressure North Korea.
Mr. Trump said he viewed North Korea, which tested an intermediate-range missile on Tuesday, as a “big problem.” But he offered no remedies. Similarly, he vowed to send a message to Iran, which is backing pro-Assad militias in Syria and which he said had benefited from a “one sided” nuclear deal with the United States negotiated by the Obama administration. But he did not say what form it would take.
At times, the Trump administration has seemed at a loss for words in responding to fast-moving events. When North Korea launched its missile, Secretary of State Rex W. Tillerson issued a statement so cryptic that it left much of Washington confused.
“North Korea launched yet another intermediate range ballistic missile,” the statement said. “The United States has spoken enough about North Korea. We have no further comment.”
Until this week, North Korea and Iran both figured higher on Mr. Trump’s list of pressing foreign problems than Syria’s civil war.
In September 2013, when Mr. Obama confronted a chemical weapons attack not unlike the one Mr. Trump faces today, Mr. Trump said on Twitter: “President Obama, do not attack Syria. There is no upside and tremendous downside. Save your ‘powder’ for another (and more important) day!” As a candidate, Mr. Trump said repeatedly that forcing Mr. Assad out of power was not as urgent a priority for the United States as vanquishing the Islamic State.
Nothing, it seems, affects Mr. Trump’s judgments as much as what he sees on television. On Wednesday, he said the images of death inside Syria affected him, presumably in ways they did not under similar circumstances four years ago. “I will tell you that attack on children had a big, big impact on me,” he said. “That was a horrible, horrible thing.”
Mr. Trump has declined to define what kind of humanitarian crisis would prompt him to act. If he considers military action in Syria, he is likely to face the same reality Mr. Obama did: While it is possible to bomb Mr. Assad’s warplanes, runways and military installations — something some senior members of the Obama administration now wish they had done — any longer-term solution would require a major presence of troops and air power.
Despite his earlier advice to Mr. Obama not to act, Mr. Trump now says his predecessor missed an opportunity to solve the Syria conflict by failing to enforce his “red line in the sand.”
“When he didn’t cross that line after making the threat,” Mr. Trump said, “I think that set us back a long ways, not only in Syria, but in many other parts of the world, because it was a blank threat.”
Mr. Trump was similarly withering about Mr. Obama’s nuclear deal with Iran. He hinted that because Congress had not ratified the accord, the new administration could somehow unravel it. The agreement, however, is not a treaty and thus does not require congressional ratification. Privately, White House officials have said the president is unlikely to rip it up.
But Mr. Trump said he did plan to deal with Hezbollah, which is backing the Assad government in Syria, and with other Iranian-backed militias that are fighting in Syria and Iraq. “You will see,” he told a reporter. “They will have a message. You will see what the message will be.”
On Thursday, Mr. Trump will face perhaps the most complex diplomatic challenge of his presidency in playing host to Mr. Xi at his private club in Palm Beach. He plans to make North Korea the centerpiece of the meeting, pressuring the Chinese to do more to compel the North Korean dictator, Kim Jong-un, to give up his nuclear weapons.
Mr. Trump has never publicly addressed the central conundrum: While he wants North Korea to give up its nuclear weapons, he does not want to open negotiations. That means he will either have to commit to using force or publicly back down by entering into another set of talks — two options his administration has found unpalatable.
Apart from some messages on Twitter and Mr. Tillerson’s own statements during a trip to Seoul, South Korea’s capital, two weeks ago — when he said the United States would negotiate with North Korea only after it gave up its weapons and missiles — the Trump administration has said very little about North Korea, quite deliberately.
Mr. Tillerson has made clear he will be a diplomat of few words, preferring to do his deals behind closed doors and open himself to as little probing of the strategy as possible. But in the absence of much public comment, American allies seem confused about the Trump administration’s strategy of coercive diplomacy.
“The conundrum,” said Robert S. Litwak, the director of studies at the Woodrow Wilson International Center for Scholars, “is that North Korea never acts except under pressure, but pressure never works.”