Why did the Japanese expand so much during WW2?

Basically I just watched the Japanese history documentary in the "Histories of the world" series on Netflix. At one point he showed a map of the empire during WW2 and it was huge. I have always wondered why they were willing to expand so much, and then fight the US to a standstill. I mean the Emperor could have just signed a peace treaty and they would not have to worry about fighting the US, plus they would have their own island nation secure. I guess I don't understand why they continued fighting and expanding. Any thoughts?