President Moon Jae-in of South Korea might have been gleeful meeting North Korean leader Kim Jong Un in Pyongyang on Tuesday. But Americans shouldn't be.

After all, this meeting is just the latest signal that the South Korean leader has abandoned the fundamental U.S. principle for negotiations with North Korea: maximum economic and diplomatic pressure that reduces alongside verifiable denuclearization and disarmament. Instead, supported by Chinese nudging and subdued by the belief that rising tensions with North Korea risk South Korea's very survival, Moon is pushing appeasement.

The broader challenge here is that U.S. and South Korea are increasingly divided over what a good deal with Kim would look like. Where the U.S. believes Kim is an opportunist who must be corralled into concessions via sanctions and the credible threat of military force, Seoul believes Kim is best constrained by the alleviation of its perception of his fears. To that end, South Korea favors increased economic support for Kim’s regime alongside a decrease in the joint U.S.-South Korean military force countering it. Moon believes that this action would ameliorate Kim’s interest in building military capabilities – such as nuclear weapons – that can threaten South Korea and the U.S.

But there’s a problem. Since President Trump met with Kim in Singapore on June 12, the North Korean leader has only continued his nuclear weaponization efforts. Why, then, would Seoul's kindness alter his mindset?

This isn’t to say that Moon’s approach to dealing with Kim is irrational. After all, Moon knows that the North Korean leader could wreak havoc on South Korea with his conventional forces even if he gave up his nuclear forces. That leaves Seoul forced to choose between focusing on the American interest in disarming Kim of his nuclear intercontinental ballistic missiles, and reducing the likelihood of a North Korean attack on its own territory. Moon has evidently decided that it is better to keep Kim happy, even if that choice leads to Kim's ever-increased threat to the U.S. It's zero-sum realism.

Of course, the U.S. cannot accept this South Korean strategy. To do so would be to endorse a North Korea that is both armed with a credible nuclear ICBM strike capability and with the benefits of an economic opening to the world. Regardless of what that would mean for South Korean (and I would suggest it would be far worse for Moon than he assumes), it would pose an utterly unacceptable threat to America. Trump should thus make clear to Moon that while he welcomes his friendship with Kim, any decisions flowing from that relationship will not bind the U.S. Instead, Trump should argue that any long term deal binding America would need to include the verifiable and durable dismantling of North Korea’s nuclear strike forces. To accept any other deal is to accept gross U.S. insecurity.

And Trump cannot accept that.