President Trump has not been shy in saying the United States could go it alone in dealing with North Korea. Raising the alarming specter of a second Korean War, the president has effectively affirmed Secretary of State Rex Tillerson’s warning that unilateral military action to eliminate North Korea’s nuclear arsenal has not been ruled out.

North Korea is on the agenda as Mr. Trump hosts President Xi Jinping of China this week at his Florida estate. Could Mr. Trump’s hints about his North Korea policy show that he plans to enlist Beijing in ousting the country’s leader, Kim Jong-un? And was the missile attack against a Syrian airbase, launched just after Mr. Xi and Mr. Trump met on Thursday evening, meant as a message to the Chinese that Mr. Trump is willing to use force unilaterally — and suddenly — against a tyrannical leader?

Unilateral American military action against North Korea would be politically foolish and militarily disastrous. China would almost certainly intervene, setting up a fraught confrontation with the United States. Without collaboration with Beijing, Washington could not achieve a surgical strike to eliminate Mr. Kim or his nuclear arsenal.

It may seem far-fetched to imagine the United States, with or without China, forcing out the regime in Pyongyang, especially while Washington is still dealing with the aftermath of ousting Saddam Hussein in Iraq and Muammar el-Qaddafi in Libya. But after years of ineffective diplomacy, American strategists would be irresponsible if they failed to explore bringing about the end of the nuclear-armed North Korean regime. The Obama administration reportedly planned for such “collapse scenarios.”