A pilot inspects a Japanese F-15 fighter jet before it takes off from the Naha Air Base in Naha, Okinawa, Japan. (Ko Sasaki/The New York Times)

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.



Under its nationalistic prime minister, Shinzo Abe, Japan has embarked on the most sweeping overhaul of its defense posture in recent memory. Not only has Abe reversed a decade-long decline in military spending as part of what he calls "proactive pacifism," but his government is also rewriting laws to lift restrictions on Japan's armed forces, which are taking a more active role as far afield as the Gulf of Aden.



It was, in fact, a speech by Abe that included tough statements on the Islamic State and an aid package to fight extremism that the militants cited as the reason they beheaded two Japanese hostages in January. Videos showing the men's bodies, posted online, gained Abe some traction for his notion that Japan must be more prepared to take on those who mean it harm.



At the heart of Abe's strategy is a drive to create a more public profile for Japan's military, the Self-Defense Forces, which have been strictly limited to defending the Japanese homeland since their creation in 1954, and which for decades afterward were barely acknowledged by a public leery of anything resembling Japan's World War II era militarism. Although Abe still does not have enough public support for his long-stated goal of constitutional changes to permit Japan a full-fledged military, he is pushing Japan's purely defensive armed forces into an unfamiliar role as the standard-bearer of a more assertive foreign policy, and a deterrent against a modernizing Chinese military.



"Japan is saying, 'Uh-oh, maybe with a rising China we have to start thinking differently,'" said Sheila A. Smith, senior fellow for Japan studies at the Council on Foreign Relations in Washington. "For the first time since World War II, Japan is finding itself on the front line. And for the first time, it has to ask itself, what does an independent defense plan look like?"



Rebuilt after Japan's defeat in 1945 at the encouragement of the United States, the country's technologically advanced military took a secondary role to U.S. forces, helping patrol strategic sea lanes in the face of a Cold War-era Soviet threat.



The Self-Defense Forces' role has expanded over the decades - Japan sent 1,000 noncombat support troops to Iraq in 2004, its biggest overseas deployment since World War II - even though the country still bars itself from possessing offensive weapons like cruise missiles considered necessary to launch full-blown attacks. With a quarter of a million uniformed personnel, Japan has slowly built up a military larger than that of other midlevel powers like France or Israel, though still far smaller than the 2.3 million-strong People's Liberation Army in China.



Just how far the Self-Defense Forces have come is evident here in the islands of Okinawa, where Japan's armed forces have been assigned a more demanding - and publicly visible - mission.



The Naha base is just a 20-minute flight by fighter jet from disputed islands that Japan controls, but China claims as its own. The islands, known as the Senkaku in Japan and the Diaoyu in China, have provided the kindling for smoldering resentment between the countries.



As China has stepped up the pressure in recent years by sending more planes and ships to patrol the islands, Japan has scrambled jets to shadow potential intruders and deployed advanced E-2 radar planes with huge dishes mounted on top to keep tabs on the Chinese while it builds a radar station on nearby Yonaguni island, Japan's first new base in decades.



The tug-of-war over the islands is a proxy for a much larger battle over the shifting power balance in Asia, where China has begun to overturn the century-long supremacy of Japan, its ancient rival.





Chinese military planners have called the Okinawan islands, including the disputed ones, part of China's "first island chain" of defense, meaning that they hope to eventually control the waters west of Japan where the United States and Japan have long held sway.While low-growth Japan is aware that it cannot match China's rapidly expanding military spending, it is trying to position its Self-Defense Forces to thwart China from trying to snatch the disputed islands, as well as to deter any designs on other Japanese-held islands.The legal changes Abe's government is working on would further free the military to come to the aid of an ally under attack, part of a broader strategy to turn Japan into a fuller military partner of the United States to try to ensure that Washington will come to Tokyo's aid if fighting breaks out over the islands.Defense analysts and U.S. commanders agree that Japan's strongest asset is its Maritime Self-Defense Force, or MSDF, widely regarded as the world's second-most capable navy after the U.S. Navy. With a tradition dating back to Japan's formidable wartime fleet, and top hardware like the Aegis radar system, the Japanese have the only naval force, except perhaps Britain's, with the ability to work so fully and seamlessly with the U.S. fleet, U.S. commanders say.This was apparent during naval war games in November involving almost 30 Japanese and U.S. warships. As the huge U.S. aircraft carrier George Washington launched F-18 jets, its closest escort was the Japanese guided-missile destroyer Kirishima. For the first time during such a complex exercise, a Japanese admiral was in charge of both navies' defense against simulated seaborne attacks."The MSDF is the most capable maritime ally that we have," said Vice Adm. Robert L. Thomas Jr., commander of the Japan-based 7th Fleet.While China's navy added its first aircraft carrier in 2012, defense analysts say Japan still enjoys a decades-wide advantage not only in technology but also in experience operating large warships. Japan has more of these larger, blue-water vessels like destroyers, and some of the world's stealthiest submarines.Last year, Japan launched its largest warship since World War II, the Izumo, a small aircraft carrier capable of carrying vertical-takeoff jets. The Izumo is part of a more mobile military that Japan is building to defend its far-flung islands to the south, including the contested ones - with or without the United States, if necessary.Still, analysts say, time is on China's side, as its economic growth rates allow ever larger military budgets. While Japan's defense budget rose 2.8 percent to a record 4.98 trillion yen (or $42 billion) in 2015, China announced Thursday that its own military spending would jump 10.1 percent in the same year, to an estimated $145 billion."The more the U.S. and Japan will do, the more China will do," Shen Dingli, associate dean of the Institute for International Studies at Fudan University in Shanghai, wrote in an email.Here at the Naha Air Base, the Japanese pilots said they tried to keep their edge with constant training. On a recent morning, they sent up a pair of F-15s to respond to a simulated intrusion, played by three other F-15s.

© 2015, The New York Times News Service

A growing number of Chinese aircraft over the East China Sea is also keeping Naha busy, so much so that the base plans to add a second F-15 squadron this year. In a nine-month period that ended in December, its pilots scrambled 379 times to intercept foreign aircraft - a sixfold jump from those same months in 2010."Every year, China's operational capabilities seem to be rising," said the Naha base commander, Maj. Gen. Yasuhiko Suzuki. "Every year, our level of anxiety rises along with them."