US' ability to control Japan has abated
Because of bitter historical memories, China has long been wary about a Japanese military revival. Indeed, even during the chilliest days of the Cold War, China provided mixed signals about the US military presence in East Asia.
Although Washington's motive to contain communist (including Chinese communist) influence was evident, and therefore resented, Chinese leaders also seemed to believe that America's supervision restrained Tokyo and prevented its rise as a strong, independent military power.
In recent years, US leaders have gradually become increasingly receptive to Japan playing a more active security role. The previous ambivalence faded noticeably during the George W. Bush's administration, and that trend has continued throughout the Barack Obama years. The implicit focus of the rebalancing strategy and other elements of US policy in East Asia is to contain China's growing power and influence.