The Ulchi Freedom Guardian exercise last year ran for 11 days, and involved 17,500 American forces, including about 3,000 from outside the peninsula, and 50,000 South Korean troops. The exercise includes computer simulations carried out in a large bunker south of Seoul intended to check the allies’ readiness to repel aggressions by North Korea.

It is possible that the event this year would be contained to so-called tabletop exercises, which would be less visible but stop short of a cancellation. Two Defense Department officials said the large-scale performances of war-fighting skills — including naval warships and high-profile maneuvers — are all but certain to not happen this year.

However, the officials said, it is also possible that even the low-key tabletop exercises will be too much for Ulchi Freedom Guardian in August. Mr. Trump’s assertion that he was canceling “war games,” the officials said, makes it hard to carry out the exercise in any form without the risk of Mr. Kim accusing the United States of not keeping its word.