But few of the military options are straightforward, and some former Pentagon officials involved in war planning for North Korea pointed to the complexities.

A major consideration would be whether and when to evacuate American and other allied civilians, which is no small feat as Seoul, a city of about 10 million, is within range of North Korea’s rockets and artillery and the North Korean military is also armed with chemical and biological weapons.

“With all this talk, what I worry about is a serious miscalculation,” said James D. Thurman, a retired Army general who served as the top United States commander in South Korea from 2011 to 2013. “Before we start talking about all these military options, we have to decide what are we going to do with the U.S. citizens over there.”

He estimated that at least a quarter-million Americans would have to be moved.

If the United States was prepared to go beyond a limited strike, it could conduct a surprise attack on North Korea’s missile garrison and weapon storage areas, using American aircraft stationed in Guam, in Japan and on aircraft carriers as well as strategic bombers that would be refueled in flight.

American officials, however, do not have high confidence that the military could find and destroy North Korea’s entire arsenal of long-range missiles and nuclear warheads. It would be up to American missile defenses to knock out any that survived and that North Korea might use to attack the United States or its allies.