With the growth of, not only China, but of Korea and other south east Asian nations, Japan is no longer in the position it once was. Japan must join free trade agreements lest they find themselves out maneuvered economically. There is a geopolitical aspect of this as well. No other nation is more nervous about the rising belligerency of China, and also of North Korea, than Japan. As the Stratfor video below will illustrate, Japan has quietly put together one of the most robust militaries in the East. In fact, outside of the United States, they may very well be the strongest military power in the region.
There are potential ramifications far down the road that may affect the relationship between the U.S and Japan negatively, however, in the short term, say the next decade or so, this is exactly what the U.S wants. Japan's robust navy, which has always been closely aligned to the U.S since the end of WII, further complements American naval power and constrains China's in the northern pacific.
Now none of this is unexpected. The U.S has been putting pressure for greater Japanese military involvement for at least a decade now, if not longer, and there was little doubt that Japan would side with the U.S as China grew in militarily. The good news for the United States is that Japan also has interests in the South China sea, and with the relative weakness of the nations, such as the Philippines, it further compliments American military capabilities. Stratfor video below.