The Japanese invasion eventually brought an American response. The United States Army Air Forces engaged in bombing campaigns that increased in severity as airfields closer to Kiska were built, and Japanese shipping was harried by U.S. naval ships and submarines. Then, in twenty days of battle in May 1943, United States Army troops retook Attu. The Japanese consequently stepped up their fortifications on Kiska, but abandoned the island on July 28, having never finished construction of the airfield. Allied forces, unaware of the Japanese withdrawal, arrived in force on August 15. In an operation that cost 200 men to a variety of causes including friendly fire, the island was occupied. Among the items left behind by the Japanese were three disabled ships on the beaches of Kiska Harbor, one at Gertrude Cove, and a sunken I-class submarine at the site of the submarine base. Allied military changes included completion of the airfield runways and the installation of further defenses. Allied forces withdrew from the island in 1946.