"Why did Japan attack..." Imagine you are sitting on an overpopulated island, trying to build an industrial economy, have minimal resources, have militaristic traditions up the wazoo and are being boycotted by other major colonial powers.

Add in the fact that your allies in Europe seem to be doing well and are encouraging you to make a move to divert the attention of the non-Axis powers. If you don't want to be treated as a minor ally by your guys and have too much self respect to ask people you despise to give you what you need, what would you do? Sieze it, perhaps?