Attu and Kiska were captured by the Japanese on 7 June 1942 following the June 3 to 4 bombing of Dutch Harbor by carrier-based planes. The Aleuts were imprisoned in Japan while Attu was developed as a garrison.

The Japanese garrisons on Attu and Kiska were subsequently subjected to heavy American aerial and naval bombardment. The U.S. Army 7th Infantry Division assaulted Attu on 11 May 1943, capturing the island by May 30. Unexploded ordnance was removed. The soft tundra covered considerable quantities of Japanese and American ordnance, which, since World War II, has been collected and moved to two different sites on the Island.

Chemical warfare artillery and bombs were stored at Attu from July 1944 to July 1947. This included mustard (M47A2 bombs) and lewisite (M33 bombs). The chemical materiel [948 one-ton containers (61 mustard and 887 lewisite), each containing about 150 gallons of agent] remaining on Attu in 1947 were dumped at sea or burned on Attu, and 20 one-ton containers (10 mustard, 10 lewisite) were transferred to Adak. Documentation on their ultimate disposition has not been found. The sea dump site was 12 miles northeast of Attu with a depth of 11,850 feet (173 degrees 32 feet E longitude and 53 degrees 3 feet N latitude).