BY NOW, the nation has grown accustomed to — if not accepting of — President Trump’s bombast and exaggeration. Plenty of his overstatements are best forgotten. But in recent days Mr. Trump’s rhetoric, from the White House, has crossed into very serious territory about two intractable crises overseas. This is where words become dangerous if untethered to action or strategy.

In Syria, the regime of President Bashar al-Assad unleashed toxic nerve agents that killed innocent civilians, including children whose corpses were a shocking testament to the awful toll of the war. Mr. Trump’s first instinct was to blame former president Barack Obama, saying the Syrian attack was “a consequence of the past administration’s weakness and irresolution.” Leave aside for the moment that Mr. Trump repeatedly argued against military action in Syria several years ago when Mr. Obama was mulling the response to his own “red line” against the use of chemical weapons. What is most interesting now is how Mr. Trump responded to the new attack. In the Rose Garden on Wednesday with Jordan’s King Abdullah II, Mr. Trump acknowledged that “it is now my responsibility” and he added, “It 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. Many, many lines.”

This raises the question of how Mr. Trump intends to make good on his word. The Syrian war, a cauldron of complexity and misery, is not an easy knot to untie. Thursday night, the United States launched about 50 cruise missiles at a Syrian military airfield in response to the chemical attack. This is the first direct strike on Syrian forces since the civil war began six years ago. The next steps will have to be carefully considered so a spasm of reaction does not simply make things worse on a battlefield where Russia and Iran are backing the Syrian regime.

Similarly, Mr. Trump faces an escalating standoff with North Korea over its missile and nuclear weapons programs. He has recognized the dead end of Mr. Obama’s policy of “strategic patience” but not announced a new policy to replace it. Interviewed by the Financial Times, he was asked about the role of China, benefactor to Pyongyang, in slowing the nuclear and missile threat. “Well,” he replied, “if China is not going to solve North Korea, we will. That is all I am telling you.” Could he do it without China’s help? “Totally,” he replied.

That suggests a very conclusive outcome that eluded Presidents Bill Clinton, George W. Bush and Obama. On Jan. 2, Mr. Trump also declared on Twitter, “North Korea just stated that it is in the final stages of developing a nuclear weapon capable of reaching parts of the U.S. It won’t happen!” These are unequivocal declarations that raise expectations for action. Mr. Trump cannot afford to drop them like so many of his past promises.