Guam, at just 210 square miles, is a virtual speck in the vast Pacific Ocean. But the island, which will soon host 5,000 Marines, is vital to U.S. Pacific defense.

Guam’s importance was underscored Saturday by Defense Secretary Mark Esper’s visit to the Micronesian island, part of a regional island hop that included a meeting with the Japanese defense minister on Guam’s Andersen Air Force Base, which would play an outsize role in any future conflict with China.