The next Democratic debate will let Mr. Sanders fill in foreign policy blanks

A LOT has happened in the month since the Democratic presidential candidates last debated. Brussels was rocked by terrorist attacks; the Justice Department unlocked the iPhone at the center of its legal dispute with Apple; Donald Trump, the GOP front-runner, questioned the importance of NATO; President Obama visited Cuba and Argentina; frightening new projections of climate-change-induced sea-level rise were published.

Former secretary of state Hillary Clinton responded by reemphasizing national security and delivering a major speech on the subject at Stanford University late last month. Sen. Bernie Sanders (I-Vt.) largely stuck to his condemnations of Wall Street, income inequality and money in politics, and his proposals to create universal health care and education programs.

Consistency can be a virtue in politics, but so can understanding and responding to global events. We are glad, then, that the Democratic candidates will debate on April 14, just five days before the New York primary, which both campaigns are treating as crucial. We hope Mr. Sanders will take the debate as an opportunity to explain in greater depth his vision for leading the country and interacting with the world.

After the Brussels attacks, Mr. Sanders declared that the Islamic State is “a barbaric organization” that “has to be destroyed,” and he emphasized that he would destroy it by creating “a very effective coalition of Muslim nations.” That’s not a new idea: How would he succeed where President Obama has had minimal success? What would he do if that effective coalition failed to materialize — would destroying the Islamic State remain an essential goal? If so, what role should the United States play?

Ms. Clinton, like Mr. Sanders, has ruled out a large commitment of U.S. ground forces, but her response to Brussels was more detailed and more plausible than Mr. Sanders’s. She called for intensifying the air campaign, increasing support to local ground troops, creating “safe spaces” for displaced Syrians and paying attention to Islamic State outposts in places such as Libya. She offered a full-throated defense of the alliance system that has formed the backbone of U.S. defense since World War II, noting that the military, intelligence and diplomatic capabilities of NATO allies are indispensable in the fight against terrorism and other efforts. She also called for creating a national commission on encryption to examine ways to give law enforcement access to terrorist and other criminal communications without compromising data security for everyone else.

Just as it would be useful to hear more details from Mr. Sanders on this agenda, voters would benefit from understanding what trade-offs his health plan would entail, how he justifies his contention that the financial sector’s business model is “fraud,” and other specifics to back up his campaign slogans. Above all, if he is to be more than a protest candidate, he owes voters a richer understanding of his views on America’s role in the world.