Given that Japan's prime minister publicly worried yesterday about a Greece-style collapse of Japan's financial system due in part to the 2.13/1 ratio of debt to GDP and their aging population they have to support with a declining overall population... perhaps you should rethink that.
I thought the trailer was dramatic, but too simplistic in its projection of geopolitics. Japan would not surrender to Korea without an actual fight. There's too much racism between the two countries. Even if the US economy collapsed, neither China nor the EU would allow Korea to take over Southeast Asia. Heck, given the six-year time frame between the Korean unification (only possible if done non-militarily, which wouldn't happen) and the annexation of Japan, Indonesia, Thailand, the Philippines, and Vietnam would still have four more years to build up both military and diplomatic resistance. I can't imagine any scenario where a unified Korea could take over all of SE Asia without any interference from any country. If the US, China, the EU, and Russia are all crippled by economic troubles, certainly unified Korea would be as well.
Maybe it's just me, but perhaps this game is trying to feed off of the "kill all Koreans!" mentality in certain gamer chat rooms.