Marine Gen. Joseph F. Dunford Jr., chairman of the Joint Chiefs of Staff, reviews a Chinese honor guard during a welcome ceremony in Beijing on Aug. 15. (Pool photo by Mark Schiefelbein via European Pressphoto Agency)

The U.S. military is preparing to launch a major military exercise with South Korea in coming days, and faces a dangerous balancing act: How do you reassure allies in the region that you are ready for a war with North Korea without provoking an actual conflict in the process?

The annual Ulchi-Freedom Guardian exercise is scheduled for 10 days beginning Aug. 21, and will include about 25,000 U.S. troops along with tens of thousands of South Koreans. The exercise focuses on defending South Korea against an attack from the north, and each year triggers threats and rebukes from North Korea. But it comes at an especially sensitive time now, following the exchange of a series of threats between President Trump and North Korea.

U.S. Forces Korea, the command that oversees some 28,500 American military personnel on the Korean Peninsula, has no current plans to change the size, format or messaging for this year’s exercise, said Army Col. Chad G. Carroll, a military spokesman in South Korea. The mission is planned well in advance, considered defensive in nature, and allows both military forces and civilian officials to strengthen their readiness for a crisis, he said.

“We will see increased numbers [of troops] on the peninsula, but no more than we see every year,” Carroll said in an email. “Our messaging will remain consistent. … These exercises are necessary to maintain readiness in the face of provocative acts threatening the [Republic of Korea] and the U.S. Our job is to provide our leadership with viable military options if called upon, and exercises like this hone our ability to do that.”

North Korea denounced the exercise Monday, warning that even an accident in the midst of it could trigger a nuclear conflict. But the war game also has drawn scrutiny this year from Russia and China, which have suggested canceling the operation to alleviate tensions. The United States has rejected that option, saying the exercise is needed to deter North Korean aggression as Washington seeks peaceful means to stop Pyongyang’s nuclear weapons development.

“This is why we have military capability that undergirds our diplomatic activities,” said Marine Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, during an appearance Monday in Seoul. “These threats are serious to us, and thus we have to be prepared.”

On Tuesday, North Korea appeared to ease up on a threat to shoot missiles toward the U.S. island territory of Guam. A state-run media outlet reported that North Korean leader Kim Jong Un said he would watch the United States “a little more” rather than responding quickly, but would “make an important decision, as it already declared,” if the “Yankees persist in their extremely dangerous reckless actions on the Korean Peninsula and in its vicinity.”

The report came hours after Defense Secretary Jim Mattis warned that if North Korea hits the U.S. island territory of Guam with a missile, it would be “game on,” meaning war.

Defense Secretary Jim Mattis said on Aug. 10 that the U.S. response to the threats from North Korea is being "diplomatically led," and he wants it to stay that way. "The tragedy of war is well enough known. It doesn't need another characterization beyond the fact that it would be catastrophic." (Reuters)

Secretary of State Rex Tillerson declined to respond directly Tuesday to Kim’s decision to pull back from his threat to launch missiles toward Guam, but said the door to talks remains open.

“We continue to be interested in finding a way to get to a dialogue, but that’s up to him,” Tillerson said at the State Department.

Tillerson and Mattis jointly host their Japanese counterparts in Washington on Thursday, with North Korea at the top of the agenda.

Secretary of State Rex Tillerson says the U.S. is interested in "finding a way to get to dialogue" with North Korea, but that it will be up to Kim Jong Un. (Reuters)

Army Col. Robert Manning, a Pentagon spokesman, said that he would not discuss specific scenarios involved in the training exercise. It remains focused on making sure that American and South Korean forces can work together well, he said. About 2,500 additional U.S. troops arrived on the Korean Peninsula temporarily last year for the exercise, according to a Pentagon news release at the time.

Manning said that the United States and South Korea have “made a lot of progress” in the last couple of years to prepare against any North Korea threat. Ulchi-Freedom Guardian is a big part of that, with two other related exercises — Foal Eagle and Key Resolve — in the spring serving as the other significant combined training events, he said.

The U.S.-South Korean military exercises have exacerbated tensions in the past. In March, the beginning of Foal Eagle prompted North Korea to test-fire four ballistic missiles, which in turn prompted the Pentagon to announce that it was assembling a missile defense system known as Terminal High Altitude Area Defense (THAAD) on the Korean Peninsula with approval of the government in Seoul.

In 2015, Ulchi-Freedom Guardian came shortly after an Aug. 4 attack in which two South Korean soldiers stepped on land mines in the heavily militarized border region with North Korea, ironically known as the Demilitarized Zone. South Korea vowed to retaliate, and the two Koreas exchanged artillery and rocket fire over the border during Ulchi-Freedom Guardian after South Korea began broadcasting propaganda messages over the border to the north and North Korea responded by turning on its own loudspeakers.

The exercise itself has changed several times, and dates back to 1968, when South Korea and the United States created a war game called Focus Lens. That occurred after North Korea hijacked a U.S. Navy intelligence ship, the USS Pueblo, and launched a bloody Special Operations raid on the Blue House, the center of the South Korean government, with plans to kill South Korean President Park Chung-hee.

Anna Fifield in Tokyo and Anne Gearan in Washington contributed to this report.