During World War II, the Battle of Peleliu was a major battle between units of the United States Marine Corps and United States Army against the Imperial Japanese Army. The battle for the island was particularly brutal because by this time the Japanese military had evolved island defense tactics with strong fortifications in the island's caves and rock formations, which enabled a defense in depth which maximized casualties on the attacking force. On both sides involved in the fighting there were high losses with more than 2,000 Americans and 10,000 Japanese killed, but, remarkably, there were no casualties among the local civilians because they were evacuated from the fighting to other islands of Palau.