Complex sentence: A more serious danger to the U.S. forces developed at sea: to destroy U.S. Navy forces supporting the Sixth Army, the Imperial Japanese Navy decided to commit nearly its entire surface fleet to the Leyte Campaign in three major task groups.
Split into simpler sentences: A more serious danger to the U.S. forces developed at sea. To destroy U.S. Navy forces supporting the Sixth Army, the Imperial Japanese Navy decided to commit nearly its entire surface fleet to the Leyte Campaign in three major task groups.