A key arms control treaty that limits the number of strategic nuclear weapons the United States and Russia can deploy is set to expire in February 2021. Without it, the two countries could be locked into a nuclear arms race not seen since the height of the Cold War. Fortunately, the New Strategic Arms Reduction Treaty, known as New START, is popular in both Washington and Moscow, and it can be extended for an additional five years with just the signatures of Presidents Donald Trump and Vladimir Putin.

Renewing it should be the easiest foreign policy decision Trump can make. However, he is holding out in hopes of getting a bigger deal, one that covers other types of nuclear weapons and also involves China. While this is a worthwhile goal in principle, focusing on it and letting the existing deal lapse could have disastrous consequences. ...