Question

How might an empire like Japan be hard to defend

Answer 1

Considered during the period of WWII I would say because the empire covered a huge extension, mainly in the Pacific, consisting of islands and atolls very far from each other and from the metropolitan territory.

Answer 2

Since they control lots of island groups which makes thing hard to border up and defend on all sides... Unlike in Germany which they had 2 fronts and it was easier to have bunkers to defend certain location...