What was the costliest battle of war?

Of the over 21,000 Japanese soldiers entrenched on the island, 20,703 died either from fighting or by ritual suicide. Only 216 were captured during the battle. The Allied forces suffered 27,909 casualties, with 6,825 killed in action. The number of American casualties was greater than the total Allied casualties on D-Day (estimated at 10,000, compared with 125,847 American casualties during the entire Battle of Normandy).[7] Iwo Jima was also the only U.S. Marine battle where the American casualties exceeded the Japanese.[8] Some 300 Navy seamen were also killed.[1]Because all the civilians had been evacuated, there were no civilian casualties at Iwo Jima, unlike at Saipan and Okinawa.[9]