As Donald Trump begins his Presidency, leaders in India face the question of whether to sign two long-delayed agreements to facilitate closer security cooperation with Washington. In New Delhi last week U.S. Pacific Commander Admiral Harry Harris offered a useful illustration of why the deals matter. Without them, he noted, India and the U.S. won’t be able to share vital information about China’s intensifying submarine presence in the Indian Ocean.

Chinese submarines have become a regular sight in the ocean in the past two...