Putin Once Tried to Curb North Korea's Nuclear Program. That's Now Over.

As Vladimir V. Putin's Russia and Xi Jinping's China deepened their confrontation with the West over the past decade, they were always united with the United States on at least one geopolitical project: preventing North Korea's nuclear arsenal from growing, or becoming more accurate. That is, until the war in Ukraine broke out two years ago. In one of the starkest back-to-the-Cold War moments yet, Mr. Putin's visit Wednesday to Pyongyang — and the announcement of a pact to provide "mutual assistance in the event of aggression" — underscored that efforts by the world's three biggest nuclear powers to halt…