The Japanese justified their imperialism in multiple different ways, depending on what territory is in question.
For China, there was a fake attack on Japanese property, performed by the Japanese themselves, so they invaded Manchuria with the justification that their citizens were in danger.
Korea and Taiwan were justified in the sense that some countries had colonies which they used for resources and as market for their products, so the Japanese justified it in the manner that they also deserve to have such territories.
For some territories they conquered there was no real justification though, and the Japanese didn't really bothered with making excuses anymore.
