China’s growing military assertiveness demands international action, and India and the U.S. are uniquely positioned to respond. The two countries aren’t treaty allies, and there’s no crisis to mobilize popular sentiment in favor of enhanced defense cooperation. Yet the imperative for military collaboration is strong. No wonder U.S. Secretary of Defense Ashton Carter, who returns to New Delhi next month, has vigorously pursued the development of an effective defense partnership over the past eight years.

With India’s vibrant...