Even if we didn't have a presence an invasion of Japan would mean war with the United States. But, there are points of friction there that could lead to something serious without our presence. There is, for instance, the disputed offshore boundary underneath which lies a massive field of Natural Gas. Japan intends to drill; China says it belongs to them.
Here's bit from globalpolicy.org:
A decision by Japan to initiate natural gas production in a disputed area of the East China Sea sparked massive anti-Japanese protests in China on April 16, the worst outpouring of such animosities in over 30 years. Although leaders of both countries sought to diffuse the crisis by promising fresh efforts at reconciliation, neither side has backed off its claims to the offshore territories. While other issues also fed into Chinese popular discontent, notably Japan's reluctance to express regret for atrocities committed by its forces in China during World War II, Tokyo's unilateral move to extract natural gas from the East China Sea was the precipitating factor. At stake potentially is the ownership of a vast undersea gas field in disputed waters lying between China's central coast and Japan's Ryukyu island chain. Because the offshore boundary between China and Japan has not been established, neither side is willing to countenance the extraction of gas by the other in the disputed "national territory." Thus, when Tokyo announced on April 13 that it would allow drilling by Japanese companies in waters claimed by China, Beijing had no compunctions about allowing an unprecedented, weekend-long display of nationalistic fervor.
http://www.globalpolicy.org/empire/economy/2005/0509energy.htm