He never specified the “bad choice.” But as George F. Kennan, the diplomat and the author of the theories for containing the Soviet Union, told students at the National War College in 1946, “You have no idea how much it contributes to the general politeness and pleasantness of diplomacy when you have a little quiet armed force in the background.” Mr. Tillerson seems to be adhering to that advice.

At the same news conference, Mr. Mattis described a situation in which the United States would act without seeking agreement from the South. If American forces in the Pacific detected a missile launch by North Korea toward American or allied soil, “we would take immediate, specific actions to take it down,” he said.

Mr. Mattis’s assertion left open the question of whether the United States would, through direct attack or cybersabotage, try to destroy North Korea’s missiles before they left the launchpad. That, in turn, could trigger a bigger operation — a plan called Kill-Chain that was named in a recent joint statement from the United States and South Korea — to systematically wipe out North Korea’s launch sites, nuclear facilities and command and control centers.

Its own authors have doubts about whether Kill-Chain could be executed swiftly enough to work, but the decision to publicly refer to it was deliberate, senior officials say. While the plan itself is classified, its goal is a systematic elimination of the North’s ability to threaten South Korea, Japan and the United States.

Among the skeptics of a pre-emptive strike was Stephen K. Bannon, Mr. Trump’s chief strategist, who was fired on Friday. Just days before, he had declared in an interview with The American Prospect, a liberal magazine, that “there is no military solution here, they got us.”

That is the conventional view. But General McMaster took issue with his predecessor in the Obama administration, Susan E. Rice, who argued in a recent Op-Ed in The New York Times that preventive war would be “lunacy.” (Preventive war describes a conflict that a stronger power starts to defeat a weaker rival and is widely considered illegal under international legal conventions. A pre-emptive strike involves attacking first when an imminent attack is detected. In American history, the debate over the two goes back 180 years, to an 1837 dust-up with Canada.)

“History shows that we can, if we must, tolerate nuclear weapons in North Korea — the same way we tolerated the far greater threat of thousands of Soviet nuclear weapons during the Cold War,” she wrote.