Moon has also criticized the U.S. military’s hurried installation of a missile-defense system in South Korea in recent weeks, scoffing at Trump’s suggestion that the South Korean government pay $1 billion for the system and calling for a thorough review of the deployment. (One of Moon’s motivations here might be his desire for better relations with China, which suspects that THAAD could be used to undermine the Chinese military’s capabilities. South Korea is caught between China, its most important trading partner, and the United States, its most important military partner.)

And Moon has responded sharply to repeated hints by the Trump administration that it may launch military strikes against North Korea’s nuclear program—strikes that could prompt the North to use artillery, chemical weapons, or even nuclear weapons against South Korea. “I want to say it sternly. Military action on the Korean peninsula cannot happen without Korea’s consent,” Moon wrote in April. He has argued that South Korea, as the country most squarely in North Korea’s crosshairs, should “take the lead on matters in the Korean Peninsula,” and that the United States and China should follow that lead.

These differences probably won’t upend U.S.-South Korean cooperation on North Korea. The alliance has weathered previous disagreements between the White House and the Blue House, including a period when George W. Bush pursued policies that resemble Trump’s and two South Korean presidents (one of them was Moon’s former boss) supported policies that mirror Moon’s.

But Moon’s election probably will constrain Trump’s options on North Korea, making the use of military force and the wholesale economic and diplomatic isolation of Pyongyang less likely. Trump may be boxed into the lengthy nuclear negotiations with North Korea that the Chinese government and South Korea’s new leader prefer.

Trump’s defense secretary, James Mattis, likes to say that war isn’t really over until the enemy says it’s over—that “the enemy gets a vote.” What Mattis and Trump might learn in the coming months is that allies get a vote too.

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