Repeated incursions into Japanese territorial waters, as Tokyo defines them, is putting a severe strain on the Japanese Coast Guard, which is the agency tasked with maintaining Japan’s control over the controversial Senkaku Islands in the middle of the East China Sea, known to the Chinese as Diaoyu.

Normally thought of in terms of rescuing sailors in peril on the sea, the Coast Guard in recent months has, in effect, become the fourth branch of the armed forces and the first line of defense in this quasi-sea battle with boats and other sea craft from China and Taiwan.

Fortunately, the confrontations have been fought with water cannons rather than real cannons. Even so, it is putting a strain on the Coast Guard, which boasts about 12,000 members and about 400 vessels of various sizes and missions ranging from buoy tenders to large, oceangoing armed patrol vessels.

The 11th Coast Guard district headquartered at Naha, Okinawa, has nine patrol cutters, but they have been augmented by drawing on vessels from other parts of Japan. By various accounts, as much as half of the patrol force has been deployed to the East China Sea to maintain Japan’s sovereignty over the islands.

At the same time, it has been called on to handle more traditional coast guard duties. September saw the Coast Guard rescue 12 sailors on a Chinese freighter that caught fire in the Sea of Japan and also rescue sailors on a fishing boat that collided with a cargo ship off Japan’s northeast coast.

For several years Tokyo has been quietly beefing up its Coast Guard, both by acquiring newer and larger cutters but also expanding the service’s duties and loosening regulations that guide their activities. Just this summer the parliament passed a law allowing the guard to arrest alleged lawbreakers rather than depending on regular police.

It has other semi-military duties such as maintaining a special anti-terrorism squad, and its mission to guard Japanese territorial waters has greatly expanded with the advent of 200-nautical-mile exclusive economic zones. Even without counting the resources surrounding the Senkaku, Japan still has far more undisputed EEZs than China or Korea.

The Coast Guard also provides Tokyo a handy way around the country’s constitutional prohibitions on maintaining an armed force. True, the provisions have not prevented Japan from raising a formidable military, known euphemistically as Self-Defense Forces, yet Japanese armament is a politically sensitive issue both at home and abroad.

The Coast Guard comes under the administration of the Ministry of Lands, Infrastructure, Transport and Tourism, not the Ministry of Defense, so it is possible to boost its budget without breaking the informal rule that defense spending not exceed 1 percent of gross domestic product. While regular defense budgets have declined in recent years, the budget for the Coast Guard has risen ten-fold in the past couple decades.

“While the [Japanese Coast Guard] has a long way to go to become a fully modernized and militarized service branch, the transformation may be the most significant and least heralded Japanese military development since the end of the Cold War,” writes Richard J. Samuels of the MIT Center for International Studies, one of the few academics who has paid much attention to the body.