HACHINOHE, Japan—It was the kind of exercise the U.S. or Japan would typically tackle by itself. On this December morning, the two countries’ soldiers were in a camouflaged tent together, practicing using the planes of one and the missile launchers of another to attack an imaginary ship of an unnamed country that might have been China.
In a forest along the coast of northern Japan, nearly two dozen U.S. Marines and Japanese soldiers hunched over maps and laptops. A message flashed on a secure Marines webpage: “Stdby for engagement.”