The defense system does a lot more good for Japan and the United States than for South Korea. Those two allies are overwhelmingly concerned with North Korean missiles. The South Koreans, by contrast, must worry about the bombardment of their capital, Seoul, by old-fashioned artillery, which North Korea has massed near the demilitarized zone in numbers that could inflict nuclear-level damage. Meanwhile, the new missile-defense system annoys China, by compromising China’s own missile force. From the U.S. and Japanese point of view, annoying China is a feature: It raises the price to China of its support for the North Korean nuclear program. But from the South Korean point of view, annoying China is more danger than help. China is South Korea’s most important trading partner. Maybe even more important, as much as they fear North Korean aggression, South Korean leaders fear even more a North Korean collapse that might thrust sudden responsibility upon them for feeding tens of millions of impoverished North Koreans. The income gap between North and South is much larger than that between the former East and West Germanies.

So … complicated. And into all this complexity stumbled President Donald Trump.

After his Mar-a-Lago meeting with Chinese president Xi Jinping, Trump stoked South Korean nationalism by seemingly endorsing Chinese claims to historic overlordship of the Korean peninsula. He later worsened things by claiming that the aircraft carrier Carl Vinson was navigating toward Korea at a time when it was in fact thousands of miles away, heading in the opposite direction. “What Mr. Trump said was very important for the national security of South Korea,” Hong Joon Pyo—one of the two leading candidates in South Korea’s May 9 presidential election—told The Wall Street Journal. “If that was a lie, then during Trump's term, South Korea will not trust whatever Trump says.”

Then, on Thursday, in an interview with Reuters, Trump denounced the U.S.-South Korea trade deal as “unacceptable" and threatened to terminate it. In that same interview, he demanded a revision of the missile-defense agreement too. “I informed South Korea it would be appropriate if they pay. … That's a billion-dollar system.”

By Sunday, cooler heads seem to have prevailed. The South Korean president’s office released a statement claiming that National Security Adviser H.R. McMaster had confirmed that the U.S. would abide by the existing agreement on paying for the defensive system. Under the “watch what the president does, not what he says” rule, everybody should now relax.

Except for two things:

1) Sunday’s words, if more responsible than Thursday’s, remain still just words. Actions usually take longer than words. If the president’s words are not guides to the president’s policy, then Americans and the world will have to live under excruciating uncertainty as they ponder whether this time the president is to be believed or not.