Canceling two annual military exercises with South Korea, President Trump has made a serious tactical and strategic error. This decision will weaken South Korea's security and make higher allied casualties likelier in any conflict with North Korea.

Predictably, President Trump and his South Korea counterpart Moon Jae-in, say otherwise. They suggest that canceling the exercises, Key Resolve and Foal Eagle, will reduce tension with North Korea with few costs to military readiness. They say that a new command post exercise, Dong Maeng, fulfills the military training benefits of the former two exercises. But they are plainly wrong.

Key Resolve is designed to prepare alliance planning staff in coordinating their response to a North Korean invasion. Foal Eagle is a field exercise which integrates allied air, land, and sea forces in maneuver training. That training better enables allied forces to regain the initiative in the early stages of any North Korean attack.

[Related: Trump: Playing war games with South Korea is 'fun' but too expensive]

To replace a field exercise with a command post exercise is an evident contradiction in terms.

And the contradiction matters, because the number of South Korean and American deaths in an attack depends on how long North Korean forces can retain the initiative. Trump is shirking his duty as commander in chief by choosing a few million dollars savings over potentially saving American lives.

But this is also a strategic failure on Trump's part. While President Moon has always wanted to appease Kim on this issue, Trump should have resisted him. After all, even though North Korea loves to wail that these exercises represent an imminent invasion plot, it knows they are defensive in nature. By giving Kim this cancellation, Trump has shown unilateral weakness absent a corollary sacrifice. That will embolden Pyongyang, which was positively shaken by Trump's withdrawal from talks in Hanoi last week.

There's a broader point here. Pursuing a nuclear deal with Kim, Trump is right to avoid irrelevant matters that upset the North Korean leader. But he is wrong to dance to Kim's military diplomacy. It only encourages North Korean hardliners to pressure Kim to play hardball. Remember, if the ultimate U.S. objective on the Korean peninsula is Kim's inability to nuke U.S. cities and conquer South Korea, Kim's objective is America's depleted footprint on the peninsula. Because that would give him the opportunity for forcible reunification.