Prior to this agreement, the United States has stood alone in warning China not to use military force to take over Taiwan, which Beijing views as a renegade province. The Japanese have been content to let the United States shoulder this responsibility. Indeed, this approach seems compelled by the Japanese constitution, in which Japan renounced the use of force.

For its part, the United States has in the past been unwilling to take any action to rekindle the embers of Japanese militarism. Now suddenly this agreement calls for Japan to take a greater role in conjunction with US forces in Asia.

So what happened?

The answer is that both the United States and Japan have begun to realize how powerful China is likely to become, both economically and militarily, as this century proceeds. And both are terrified by that prospect. To paraphrase the title of my new novel, the enemy of my enemy is my friend.

A number of recent events have led up to this agreement. On the United States’ side, our military forces are stretched thin in Iraq and elsewhere in the Middle East. Without assistance from Japan or somewhere else, we would be at a considerable disadvantage if China were to attack Taiwan. Sensing that the Chinese have understood our vulnerability, the Bush administration spent the last couple of years cajoling Taiwanese politicians to turn down their rhetoric for independence that could inflame Beijing. At the same time, decision makers in Washington have kept their fingers crossed, hoping that this was sufficient to do the job.

Economically, in Washington, government officials have watched China’s 9.5 percent annual growth rate with envy. A milestone was reached last year when China surpassed the United States as Japan’s number one trading partner. In the twentieth century, the United States leapfrogged over England as the world’s foremost economic power. Will the Chinese pass the United States in the twenty-first?

For the Japanese, conflicts with China are closer to home. The economies of both of these Asian giants are heavily dependent on imported oil and gas. Not content merely to forage in the Middle East and Africa, China has been exploring for natural gas in Japanese claimed waters. In response, the Japanese have taken aggressive action to defend their territorial claims to islands in the East China Sea.

The United States is playing a dangerous game by bringing Japan into its dispute with China concerning Taiwan. For starters, the enmity between Japan and China from the Second World War is far from settled in the manner of France and Germany. One needs merely to travel to one of these countries and raise the issue with people whom you encounter, as I have done on several occasions.

Then there is the fact that Japan occupied Taiwan from 1895 to 1945. Regardless, of what the Taiwanese thought of Japan during this long occupation, some Taiwanese leaders, who have the delusion of independence for the island, will play any card, no matter how manipulative, to achieve that goal.

Some Japanese officials have been doing their best to stir things up. Recent Japanese visitors to Taiwan, including the mayor of Tokyo, claim to be promoting tourism when in fact they have been encouraging and stoking the flames of Taiwanese independence.

Throw into this mix the fact that the Japanese economy continues to struggle as it has for the last several years. It may very well be back into recession. As Germany demonstrated in the thirties, economic frustration breeds military aggressiveness.

Hopefully, all parties will show restraint over Taiwan, but this remains to be seen. With one major spark, which the Taiwanese leadership could provide, the powder keg could blow. If that occurs, the Bush administration will face a critical decision. Will it be a participant on the Japanese side, or a referee trying to separate the two fighters?