Donald Trump on Tuesday praised Moon Jae-in for “great cooperation” on containing the threat from North Korea and said there has been “a lot of progress.” The South Korean President also made a show of unity after their summit in Seoul, but Mr. Moon’s recent actions suggest he is an unreliable friend.

Mr. Moon favors appeasing Kim Jong Un to lower tension on the Korean Peninsula, including direct talks even as the North continues its nuclear and missile tests. He wants to reopen the Kaesong Industrial Zone that provided...