Bound up with Japan and China's growing economic rivalry and competition for resources is a struggle for strategic control. For centuries, China was the dominant power in the region. It bridled at its inferiority to Japan over the last century, and now seeks to reassert its dominance.
Central to this rivalry is military power. China has reported big recent increases in military spending - and is suspected of under-reporting such expenditure. In an important recent defence report, Japan for the first time labelled China a security concern, along with North Korea.
Meanwhile, Japan continues gradually to expand its military role, in spite of its pacifist constitution. It too is one of the world's biggest spenders on defence. Its contributions to US-led military operations in Afghanistan and Iraq reflect its desire to play a more assertive role.
A major security focus is Taiwan, which China regards as a breakaway province but whose leaders argue is a sovereign state. The US has made clear it may be prepared to defend Taiwan in the face of a Chinese assault - and any such operation would most likely draw in Japan, the nearest US military foothold. Japan underlined its support when it declared Taiwan a security concern in a joint statement with the US.
Until now, Japan has relied heavily on the protection afforded by its American ally, but there are signs it may be seeking a more prominent role as an international heavyweight - such as its thus far unsuccessful bid to gain a permanent seat on the UN Security Council. Chinese protesters have opposed this campaign.
China too has thrust itself confidently onto the regional and world stage, taking a lead role in local diplomatic initiatives, repairing ties with India, joining Brazil, Russia and India in a new economic bloc of developing nations, and travelling far afield to secure new ties and trade deals.
Japanese portrayals of China may betray unease about the rapidly changing power balance in the region - such as when Japan's trade minister recently called China "a scary country".