Ms. Park was elected on a platform of reopening the possibility of warmer relations with the government of Mr. Kim, but the rise in tensions has all but eliminated that opportunity, at least for now. Under current agreements, the South Koreans remain in command on the peninsula under normal armistice circumstances, but General Thurman, as the commander of American and United Nations forces, would assume operational control if war broke out. Wartime control is set to transfer to South Korea after 2015.

Ms. Park would be under extraordinary pressure to take action if the North acted out again. When the Cheonan, a South Korean warship, was sunk in March 2010, her predecessor decided not to strike back — and it took months to complete a study that concluded the explosion aboard the ship had been caused by a torpedo shot from a minisubmarine based just over the border in North Korea. Months later, the North shelled a lightly inhabited island in the South — and was met by delayed and ineffective return fire.

“The new agreement defines action down to the tactical level and locks in alliance political consultations at the highest level,” an American official said. The official stressed that the South Korean military would take the lead in any response to hostilities from the North short of war.

“North Korea has gotten away with murder — literally — for decades, and the South Korean and American forces have rarely responded with decisive military action,” said David S. Maxwell, a retired Army colonel who served five tours in South Korea.

“It’s very important to break the cycle of provocation,” said Mr. Maxwell, now the associate director of the Center for Security Studies at Georgetown University. “These responses have to be proportional. They have to be delivered decisively, at the time and at the point of provocation.”

As part of prescheduled military exercises with South Korea, and to prove America’s commitment to regional security, the United States mounted an unusual, highly publicized show of force. It included the decision to use nuclear-capable B-2 bombers, which have a stealthy design to avoid detection, to conduct a mock bombing run in South Korea.