The IJN launched a surprise attack on Pearl Harbor, killing 2,403 Americans and crippling the U.S. Pacific Fleet. During the first six months of the Pacific War, the IJN enjoyed spectacular success inflicting heavy defeats on Allied forces. Allied navies were devastated during the Japanese conquest of Southeast Asia. Japanese naval aircraft were also responsible for the sinkings of HMS "Prince of Wales" and HMS "Repulse" which was the first time that capital ships were sunk by aerial attack while underway. In April 1942, the Indian Ocean raid drove the Royal Navy from South East Asia.