The US and Japan signed a new set of defence agreements on Monday that allow for greater co-operation between their militaries at a time of rising Chinese influence in Asia.
Part of a broader push by Japan to shed some of the restrictions of its postwar pacifism, the new guidelines would allow the US and Japanese militaries to work more closely together in the event of a conflict in the East China Sea or in North Korea.
For the Obama administration, the improved defence ties with Japan are an important part of its "pivot" to Asia which has also seen the US expand military co-operation with allies such as Australia and the Philippines and former enemies such as Vietnam.
While the administration insists that its approach to the region does not represent an effort to contain China, the thread that ties together all these different initiatives is the growing anxiety across Asia about how to deal with a more powerful China.
The new guidelines were released at the start of an official visit to the US by Japanese prime minister Shinzo Abe. The focal point of the week-long trip will be a speech to both houses of Congress on Wednesday, where every word from the conservative nationalist Mr Abe about Japanese behaviour in the second world war will be closely parsed, especially by China and South Korea.
He will also meet President Barack Obama at the White House on Tuesday, where they are likely to present an upbeat outlook for prospects for the Trans-Pacific Partnership, the 12-country trade agreement that is reaching a crunch point.
Jim Schoff, a former Pentagon official and Japan specialist at the Carnegie Endowment in Washington DC, said that the new defence guidelines would be the most lasting legacy of Mr Abe's trip to the US, despite the intense focus on the way he addresses historical issues in his speech to Congress.
"This could be a major change in how the alliance functions," he said. "The US is hoping to get Japan more deeply involved in international missions and Japan wants to bolster alliance integration as a way of increasing deterrence against China."
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The main thrust of the new defence guidelines, which call for "seamless and robust" responses to events in the Asia-Pacific, is to allow for significantly increased co-operation between American and Japanese forces which had been limited by Japan's own self-defence doctrines.
A high priority will be planning for how to respond to incidents involving their navies in the East China Sea, where Japan and China have a territorial dispute, but also to work more closely together on intelligence, humanitarian relief, missile defence and in cyber and space. The two militaries will establish a permanent co-ordinating mechanism that will be used to respond to crises.
The US and Japan established defence guidelines in 1978, at the height of the Cold War, and in 1997, when China's military build-up was still in its infancy. The new guidelines are the first to reflect China's greater military heft and push for influence in the region.
The statement from the two governments reaffirms that the US believes the Senkaku Islands, which are claimed by both Japan and China (which calls them the Diaoyu), are covered by the US-Japan security treaty. It also notes the already stepped-up US military presence at bases in Japan, including the deployment of Global Hawk drones, P-8 patrol aircraft and Aegis ships.
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