Japan's decision would be critical. If the North Koreans invaded South Korea or if China attacked Taiwan, the three U.S. Air Force bases and two major naval bases in Japan would be absolutely vital to back up any American defensive efforts. Tokyo has already indicated that it would allow the U.S. to use them to repel a North Korean attack on the South, where 36,000 U.S. troops are stationed. The Taiwan issue is tougher. Not only does Tokyo recognize Taiwan as part of China, but the Japanese also have no desire to provoke Beijing.
China is a nuclear power and right next door.
We don't have the will to face China under those circumstances. American officials try not to talk about it either, but they doubt that the Japanese would grant permission. In spite of these limitations, Clinton and Hashimoto will be trying to impress upon their home audiences that the alliance is still vital. Most other countries in the region, probably including China, support it, not least because it helps keep Japan militarily unthreatening. Meanwhile, the Japanese government is considering proposals to legislate some clarity into how it might assist the U.S. in regional conflicts, and the Americans will take steps to make life slightly less difficult for Shoichi Chibana and his fellow Okinawans.