Donald Trump’s military strike against terrorist leader Qasem Soleimani hasn’t led to war with Iran but it has opened up hostilities in the Democratic presidential race. Bernie Sanders wants to use the tension in the Middle East to tar Joe Biden for his 2002 vote authorizing war in Iraq, in contrast to Mr. Sanders’ opposition to the use of American military power. In Tuesday night’s debate, Mr. Biden will need better responses.

This weekend the Sanders campaign called Mr. Biden’s position on the Iraq war “appalling” and...