NAHA, Japan — Once a sleepy, sun-soaked backwater, this air base on the southern island of Okinawa has become the forefront of a dangerous test of wills between two of Asia’s largest powers, Japan and China.

At least once every day, Japanese F-15 fighter jets roar down the runway, scrambling to intercept foreign aircraft, mostly from China. The Japanese pilots say they usually face lumbering reconnaissance planes that cruise along the edge of Japanese-claimed airspace before turning home. But sometimes — exactly how often is classified — they face nimbler Chinese fighter jets in knuckle-whitening tests of piloting skills, and self-control.

“Intercepting fighters is always more nerve-racking,” said Lt. Col. Hiroyuki Uemura, squadron commander of the approximately 20 F-15 fighters stationed here at Naha Air Base. “We hold our ground, but we don’t provoke.”

The high-velocity encounters over the East China Sea have made the skies above these strategic waters some of the tensest in the region, unnerving Pentagon planners concerned that a slip-up could cause a war with the potential to drag in the United States. Japan’s refusal to back down over months of consistent challenges also represents a rare display of military spine by this long-dovish nation, and one that underscores just how far the rise of China and its forceful campaign to control nearby seas has pushed Japan out of its pacifist shell.