The United States' stance which was largely neutral earlier over the disputed islets in the East China Sea between China and Japan seems to have morphed from ambiguous neutrality to a more pro-Japanese flavor. Earlier the U.S. had adopted a position of calculated ambiguity verging on neutrality on this fraught issue which is seriously rolling the Chinese-Japanese relationship. What has caused the U.S. to seemingly shift its stance is both interesting and not so easily identifiable. Granted, the U.S. has treaty obligations towards its ally, Japan, which mandates the former coming to the aid of Japan when the latter was militarily attacked. This has not happened yet. While the Chinese has declared an air-exclusion zone across the East China Sea -- which has recently been breached by U.S. aircraft based in Guam in the Pacific without much Chinese reaction -- it has not materially ratcheted up tensions between the two arch-rivals, China and Japan.
There was a time when Japan was the second-largest economy in the world behind the United States. Years of a dispiriting recession have put paid to that exalted status. Now in the second decade of the 21st century, it has to cope with the reality of a resurgent China poised in the next few years to overtake the United States as the world's largest economy. Meanwhile, Japan has slipped in the largest economy tables to go down the list below Germany. It is not easy for Japan to adjust to this reality. Japan, in the 20th century, had exhibited a kind of superiority complex vis-à-vis both China and Korea. When the militaristic War Party was ruling the roost in Tokyo, the Japanese, apart from occupying Manchuria, also invaded and occupied parts of China. The Chinese remember the Rape of Nanking and other Japanese depredations in the 1930s. Reportedly, millions of Chinese lost their lives through Japanese operations in China. Historical memory lasts a long time. Moreover, the Japanese and other countries such as the Philippines and Vietnam among others, have not adjusted fully to China's emerging position as a regional hegemon in South East Asia.
The United States should act as a mediator between China and Japan. Overt support for Japan may not prove to be an optimal position in the long run for the U.S. as it tries to pivot to the Asia-Pacific region after its travails in the wider Middle East. According to the Washington Post, the U.S. is heavily indebted to China, which holds around one quarter ($1.28 trillion) of the U.S.' total foreign debt amounting to $5.6 trillion. I do not think it would be politic for a debtor nation to take an overtly anti-Chinese position in these circumstances. Also the U.S. and China have a huge trading relationship albeit skewed heavily in favor of Chinese exports to the U.S., which could be adversely affected if the above eventuality comes to pass.
One does not have to be an expert on Chinese-U.S. dealings to determine quickly that common sense dictates against such an approach. Instead, a measured statesman-like approach calling on both Japan and China to avoid escalating tensions over the islands would redound to the U.S.' credit. Ideally, of course, if both China and Japan were to take their dispute to the International Court of Justice (ICJ), having mutually agreed in advance to abide by the Court's verdict, the Diaoyu/Senkaku issue could be resolved peacefully and amicably. Quite often, large and powerful countries with an ego to match, do not wish to submit themselves to a verdict of the ICJ. However, some years ago, Bahrain and Qatar used the ICJ mechanism to resolve their long-standing dispute over the Hawar Islands, and recently Chile and Ecuador also sought the help of the ICJ to resolve their dispute over maritime limits.
It bears repetition that the ICJ was instituted as a principal organ of the post-WWII U.N. system to help countries resolve their conflicts through the impartial judicial intervention of the ICJ. If only more countries had shown wisdom to resort to the ICJ, the world would have been a safer and more peaceful place.
CORRECTION: The terminology "South China Sea" in a previous version of this post has been revised to "East China Sea." This post has been updated accordingly.