If former vice president Joe Biden wins the Democratic presidential nomination, his success might be attributable to two factors: his defense of President Barack Obama’s record and his foreign policy views. He showed off both Wednesday night.....

Biden accomplishes two things here. First, he reaffirms his bond with Obama, who remains overwhelmingly popular with Democratic voters, most especially with African American voters. Certainly, Biden benefits from association with a popular ex-president, but he also reminds Democrats that unlike his opponents who want “revolutionary change," he wants to build on the Obama legacy. Second, Biden’s analysis is essentially correct, and his rivals would be foolish not to adopt it....

It is one thing to criticize Trump for making hash out of our Middle East policy, however, and quite another to construct a coherent plan for improving matters. What is the endpoint we seek? If we aim to renew the Iran deal, decrease Iranian aggression in the region and maintain vigilance against the Islamic State, then immediately bugging out of Iran, Syria and Afghanistan — as some of Biden’s opponents suggest — would be foolhardy. Simply reciting platitudes such as “Get out of the Middle East” does not engender confidence among voters and frankly does not present much of a contrast to Trump.

Biden offers something more concrete and realistic: Keep allies together, use carrots and sticks to get back to an acceptable nuclear deal and attempt to ratchet down the chance for war. That requires a whole lot of finesse, which is not amenable to bumper-sticker promises: Work with our allies. Don’t govern by tweet. Use diplomacy. That’s as simplistic and unimpressive as “Iran won’t get a nuclear weapon on my watch.” Biden opponents’ one-liners or one-upmanship about who opposed the Iraq War first will strike many voters as childish.

Candidates need to show that they understand the challenges they will face going forward. One of them might inherit the Trump legacy of chaos, instability, broken alliances and disdain for human rights.