When President Donald Trump agreed to pause military exercises between the U.S. and South Korea on Tuesday, he gave Kim Jong Un a concession on an issue that has angered North Korea like few others.

Pyongyang has long said it views such joint drills as preparation for an invasion.

The president's revelation that he planned to put the "war games" on hold was part of the agreement with Kim to work "toward complete denuclearization of the Korean Peninsula."

"Under the circumstances that we're negotiating a very comprehensive complete deal, I think it's inappropriate to be having war games," Trump told reporters.

He also described the exercises as "very provocative" and said the move would "save us a tremendous amount of money."

The next round of the joint drills were due to be held in August.

But both the Pentagon and South Korea's military appeared to have blindsided by Trump's statement.

U.S. Forces Korea said it had "received no updated guidance," and Seoul said it would need to clarify the "intention behind his comments."

Adam Mount, a senior fellow at the Federation of American Scientists, suggested Trump had offered more to Kim than he needed to.

Ending US-ROK exercises is in excess of all expert consensus, South Korean requests, and even a close reading of North Korean demands. Trump thinks of vitiating the alliance as a goal, not a concession! https://t.co/lSnIR0lX6F — Adam Mount (@ajmount) June 12, 2018

Around 28,000 U.S. troops are based in South Korea. The U.S. and its allies maintain the drills are purely defensive in nature.

As well as putting the North on edge, the drills highlight the gulf between the capabilities of the South Korean military and its advanced U.S. ally, and the sizable but aging North Korean forces.

In December, joint exercises named Vigilant Ace involved some 230 planes and 12,000 U.S. military personnel in what was described as "a realistic air combat exercise."

Pyongyang saw things differently, labeling the drills as a "grave provocation" that threatened to escalate the region "to the brink of nuclear war."

In contrast to Tuesday's cordial back-patting, at the time North Korean state-run media said "insane President Trump is running wild" and condemned South Korea as "puppet war maniacs."

It was far from an isolated incident. In 2016 North Korean media threatened a "pre-emptive nuclear strike of justice" on Washington and Seoul in response to that year's drills.

"If we push the buttons to annihilate the enemies ... [they] will be reduced to seas in flames and ashes in a moment," the North's National Defense Commission said in a statement.

This animosity toward the American war machine stems from the Korean War, which has never officially ended despite an armistice in 1953.

While anti-American sentiment has certainly been fomented by North Korean propaganda over the years, it is based in some fact. During the Korean War, the U.S. Air Force carried out some of the most intense saturation bombing ever seen.

"The tonnage of bombs dropped on the North was about the same as the total dropped by the U.S. against Japan during World War II," The Associated Press reported last year. "North Korea is probably second only to Cambodia as the most heavily bombed country in history."

The U.S. dropped 635,000 tons of bombs on Korea, most of it in the North, including with 32,500 tons of napalm, the AP said.

Estimates vary, but according to the Encyclopedia Britannica, 600,000 North Korean civilians and 400,000 North Korean military personnel were killed. Add to this about a million South Korean civilians, and 200,000 South Korean, 36,500 U.S. and 600,000 Chinese military personnel.

Despite signing the statement with Kim, Trump told reporters Tuesday that reducing the number of U.S. troops in South Korea was "not part of the equation right now."

Sen. Lindsey Graham, R-S.C., told NBC News' "Today" that he didn't think canceling war games would "matter over the arc of time."

He added: "Kim comes out of this thing bigger. The U.S. comes out stronger, too."

However, Graham added he would "violently disagree" with any proposal to remove U.S. troops from South Korea.