Complex sentence: The island now forms the Baker Island National Wildlife Refuge and is an unincorporated and unorganized territory of the U.S. Its defense is the responsibility of the United States; though uninhabited, it is visited annually by the U.S. Fish and Wildlife Service.
Split into simpler sentences: The island now forms the Baker Island National Wildlife Refuge and is an unincorporated and unorganized territory of the U.S. which vouches for its defense. It is visited annually by the U.S. Fish and Wildlife Service.