A mock war erupts on the Korean peninsula each year, largely in secret, with thousands of military troops and their commanders tracking and repelling a North Korean attack.
It's all a drill, a large-scale theoretical exercise aimed at protecting democratic South Korea, a U.S. ally.
But North Korea is not a fan.
The military exercises — known as Ulchi-Freedom Guardian — have been in the works for months, and began Monday. That prompted the totalitarian nation to declare the drills were "like pouring oil over fire," and "aggravating" an already tense situation on the peninsula.
The war games, which involve thousands of troops from the United States and South Korea, come at a time when Washington and Pyongyang have seemed perhaps on the verge of an actual war over the North Korean government's rapidly advancing nuclear and missile programs and the bellicose language traded by leaders on both sides.
North Korea's repeated objections over the years about the exercises have also prompted at least two powerful countries in the region, China and Russia, to suggest halting them as part of a potential deal to get nuclear concessions from Pyongyang.
Talk of such a freeze has been a non-starter in Washington.
Former military commanders who worked in South Korea say the drills, held since at least 1976 under various names, keep U.S. forces ready in a foreign theater that has annual turnover of rank-and-file service members, their officers and the civilian analysts who support them.
"Practice is needed because training is perishable for everyone," said David Maxwell, a retired Army colonel who participated in the exercises during five tours on the peninsula. "So many of the forces are rotating from Iraq and Afghanistan. They've got to be quickly educated on the Korean problem, which, of course, is much different."
This year's drills come at a particularly perilous time.
The North last month successfully test launched two intercontinental ballistic missiles that could, in theory, strike the American mainland, a technological advancement that has long worried security officials.
The exercises also began days after President Trump said North Korea would "be met with fire and fury like the world has never seen" should its threats continue.
And the Kim Jong Un government also recently threatened to launch missiles near the strategically important U.S. territory of Guam, a tiny Western Pacific island home to Andersen Air Force Base.
The drills, named for a famous historic Korean military leader, are confined largely to command posts and computers, not live weapons. They allow military leaders to practice complex scenarios in a relatively short period of time without large-scale troop movements, according to those who've participated. There are also drills in March that involve more live-fire action.
The exercises have, at times, prompted serious responses from the North.
This time last year, the North test-launched a submarine-based ballistic missile from its eastern sea base in Sinpo. A week after those drills ended, the government detonated a nuclear device underground from its Punggye-ri test site — one of three illicit tests under the rule of Kim Jong Un, whose grandfather, the late Kim Il Sung, is the country's communist patriarch.
Such tests violate resolutions by the United Nations, which has also applied economic sanctions on North Korea in response to its tests.
The North is considered a nuclear state with an advancing ability to miniaturize its weapons and deliver them with long-range missiles. Stopping its momentum has perplexed U.S. leaders for several presidential administrations, which have tried pressure, economic aid and reportedly covert action at various times — all with little success in curbing the nation's nuclear ambitions.
The exercises comply with the terms of the 1953 armistice between the two nations that halted the Korean War, observers say, and shouldn't create misunderstood or surprise threats for the North — despite its vocal objections.
"Pyongyang knows very well they are defensive and knows when to expect them," said Duyeon Kim, a visiting senior fellow at the Korean Peninsula Future Forum in Seoul.
South Korean President Moon Jae-in, a former soldier, told his cabinet Monday that the exercises were "an inspection of our defense posture, for the protection and safety of our citizens. … There is no intent to escalate military tensions in the Korean Peninsula."
Secretary of Defense James N. Mattis told reporters that North Korea knows the exercises are defensive.
"For whatever they may say for public consumption, they know this is a defensive exercise," Mattis said. "It's been going on, you know, for decades."
One idea floated recently by China and Russia suggested that the United States stop its exercises, including those in the spring, which have prompted outrage in Pyongyang over the years. In exchange, the theory goes, North Korea might pledge to halt its nuclear and missile testing.
Top U.S. military commanders, including Gen. Joseph F. Dunford Jr., chairman of the Joint Chiefs of Staff, have rejected such a move. It's also unclear whether Pyongyang would go along.
"Why would anyone call off an exercise in hopes of getting better behavior out of the Kim family regime?" said Grant Newsham, a senior research fellow at the Japan Forum for Strategic Studies, recalling a generation of past overtures that ultimately failed.
Others say offering to halt the exercises — or at least trying to downplay them — might help convince the North to stop its tests, especially in the absence of better options.
John Delury, an associate professor of Chinese Studies at Yonsei University in Seoul, said the exercises serve a planning purpose, but they also have a "ritualistic element" that touts and dramatizes the United States and South Korea's combined military might.
"If the North Koreans are willing to give stuff up, and that's what we really want, then we need to explore that," he said. "I'm pretty sure that our military can figure out ways to ensure that troops are trained and ready — and the interoperability [with South Korean forces] is there."
This year's operation is scaled back, though perhaps not because of recent tensions.
The participation of local United States forces is down to about 17,500 from 25,000 last year, though officials say recent events aren't responsible for the size. South Korean media also reported fewer of its troops were involved, though United States officials declined to discuss those figures.
U.S. forces generally don't publicly discuss the operational details of the exercises, but they involve computer-simulated scenarios, both in the lead-up to and defense against, a North Korean attack.
The operation, generally speaking, splits into two parts, involving top-level decisions about preparedness and ground-level work to repel a theoretical threat. Planners spend months preparing possible North Korean moves, and they consider its military capabilities.
They then use computers to simulate how events might unfold. Troops and commanders respond to the simulations, and computers gauge the results, former participants say.
Although much of the heartburn about North Korea in recent years has centered on its advancing nuclear and missile programs, what gets less attention are the thousands of conventional weapons — like rockets and artillery — that can be used against the South.
Seoul, a metropolitan area of 25 million, is about 45 miles south of the border, well within range of some of those systems. A conventional attack by the North could kill tens of thousands in the South within hours or days.
Such an attack on civilians could eventually also invite a potentially fatal response to the Kim Jong Un government from the United States and South Korea.
It's precisely the type of scary scenario United States and South Korean military brass might be defending against this month, said Maxwell, the retired army colonel, now an associate director at Georgetown University's Center for Security Studies.
"They are trying to anticipate everything that North Korea would do that we have not thought about," Maxwell said.
Stiles is a special correspondent.
4;10 p.m.: This article was updated with a comment by the secretary of Defense.