it refers to Geopolitical theory, well, I think if Chinese have Geopolitical theory, it is totally different from the US, Britain, and Japan.China is a mainland country, not like these island(North America is a big island surrounded by Pacific Ocean and Atlantic Ocean). So Chinese view geography from a very different perspective: it is surrounded by different countries, in other words, different threats, during its 4000-year-history, it has been invaded and conquered by outsiders for hundreds of times. And every time, these invades causes terrible hurts.
Therefore, geopolitical security is the core of China's foreign policy, and I think that why in some cases, China act "aggressive".
China's terrain is also very strange, the Western China is very high, and the Eastern China is relatively low, the North is high and the South is low, so in history, most of invaders came from the Northwest part, for the central governments, the mountain and highlands in west is natural barrier to the invaders(and they built the Great Wall to protect the poor agriculture residents from nomadic tribes).That's why China is always alert to its western province,include Xinjiang and Tibet, as well as the big neighbours like Russia,India and Turkey.
The south China sea is another core interest for China, Taiwan, is the most important. for China, it is a door open to the Pacific, but now it is the crucial part of the first island chain. Taiwan will unquestionable return to China, but I think the probability of war is getting smaller and smaller. The Chinese government has too many peaceful methods to reunify Taiwan, it is foolish to use expensive missiles and ships.
China claims that the South China Sea is the "inherent territory", but I think this dose not means that neighbouring countries can not fishing or shipping in this water. The Chinese government hope to keep the sovereignty, in that the south China sea is economic lifeline and energy pathway.
Japan, Oh, for Chinese people it is historical hates, but in geopolitics, Japan is not so important, in history Japan invaded China twice, in the first time it is defeated by China and Korea, in the second time(the WWII), it brought about maximum pain and terrible memory. I think it is very hard for American people to imagine the suffering. The war is a genocide.The Chinese government do no think Japan is a big threat, but a disgusting threat.
Well, I'm not native speaker so please excuse my poor English. If you can point out the grammar mistake, I will be very grateful.