The tiny island of Tinian was the launch point for U.S. planes carrying atomic bombs to Hiroshima and Nagasaki in 1945. Now a new runway is being carved from the jungle, just south of World War II ruins.

And on a blustery February morning, a few hundred meters away at Tinian’s civilian airport, American airmen refueled Japanese fighter jets during a military exercise using more airstrips, islands and Japanese planes than the two enemies-turned-allies have ever mustered for drills in the North Pacific.

Asia and the Pacific are steering into an anxious, well-armed moment with echoes of old conflicts and immediate risks. Rattled by China’s military buildup and territorial threats — along with Russia’s war of aggression in Ukraine and doubts about U.S. resolve — nations across the region are bolstering defense budgets, joint training, weapons manufacturing and combat-ready infrastructure.