The Divider: Trump in the White House, 2017-2021 by Peter Baker and Susan Glasser:
Kim spun a big story about his willingness to abandon nuclear weapons, claiming to be committed to full denuclearization of the Korean Peninsula, without Trump fully grasping that meant something very different to each of them. To Kim, it meant all American troops out of South Korea and a series of steps—“action for action,” in the catchphrase of the negotiators that Kim used at the end of the summit, to Bolton’s horror—that each would take along the way. To Trump’s hardline national security team, “action for action” was unacceptable. They were looking for Kim to unilaterally give up his weapons first; only then would sanctions relief and other measures follow.
The trouble began when Kim pleaded for a gesture to help him with his own hardliners: Couldn’t Trump offer something, say, by canceling or cutting back the military exercises conducted jointly by the United States and South Korea that were a regular source of friction? Trump, who had repeatedly pressed Jim Mattis to halt them as a waste of time and money, agreed on the spur of the moment without consulting his national security team or the Pentagon. He even adopted Kim’s language, calling them “war games” instead of exercises, and thanked Kim for saving the United States a lot of money. That was exactly the concession that Vladimir Putin would want to see, an America pulling back. Matt Pottinger, the senior Asia adviser who had been in the meeting, told others bluntly that it was “a complete giveaway with nothing in return.”
Back at the Pentagon, the nation’s military leadership found out by watching cable news. “Let’s take a deep breath,” Chairman Joe Dunford told the emergency meeting that gathered in his office to figure out what to do. The major twice-a-year exercises were a key part of the deterrence strategy against North Korea and were held so frequently because of the annual rotations of American personnel on and off the peninsula; many smaller ones also took place. The White House did not formally notify the Pentagon of Trump’s decision until days later. The order was to “just cancel all exercises,” a senior defense official recalled. “We said that’s not doable. You actually have to have exercises.” Eventually, a complicated matrix was produced; exercises over a certain size would require White House approval, while smaller ones or those that were virtual could be held without permission.