That was some speech Tuesday by Joe Biden. The Democratic presidential candidate and former Vice President wrapped himself tightly in Barack Obama’s Iran policy, called Donald Trump’s strike against Iranian General Qasem Soleimani a “debacle,” and all but declared that Mr. Trump is the greatest single danger to world order. As election bets go, this is a big one.

It isn’t news that Mr. Biden thinks Mr. Trump is “dangerously incompetent.” He’s said that before. The news Tuesday is that the leading Democratic presidential contender...