Principle number 6 of Basic Elements of Islam: Lands must be conquered. Lands that Islam has lost must be reconquered (Spain and Israel, for example). The Islamic empire must continually expand. Contraction is bad; expansion is good. So if a land was once Islamic and now it is not, that's contraction, and must be remedied.
According to Islamic teachings, the earth is Allah's. If there are parts of the earth not following Islamic law, it is the duty of the faithful to gain control of that land and establish Sharia. It is a sin to let it be.