Which is the role of the capacities and the dynamic factors of the japanese state and of the southeast Asian region in the expansion of Japan between 1940 and 1943? This is the question that will lead this text. This question will me approach through the realist theory and will be divided un four chapters. The first chapter will be dedicated to explain the realist theory, followed by the Japanese case. Then we will talk about the southeast Asian region son we can have an idea of the situation during 1940. The countries that we will study in this chapter are Thailand, Philippines and Indonesia. Once we have all this information we will move forward to chapter 4, where I will make a small analysis on how all the things mention above can explain the Japanese expansionism. And finally I´ll finish with a few conclusions.