If I were to give a very general answer, this is the concept of imperialism/expansionism. Humans seek to improve themselves by nature i.e. rich people want to become richer. In the same way, Japan (and actually any powerful country) would want to expand and colonise/seize other countries to improve their global standing and become more powerful. (Imagine if Japan seized every country - we would be under their control! - just a thought haha)
