Despite the limited threat of attack, the United States military took steps to protect the B-29 bases in the Mariana Islands. The USAAF's Seventh Air Force played the most important role, and continuously attacked the Japanese-held islands until just before the end of the war. United States Navy (USN) and United States Marine Corps (USMC) aircraft and XXI Bomber Command B-29s also occasionally participated in these attacks. The Seventh Air Force also provided air defence for the Mariana Islands; this task was assigned to the 318th Fighter Group, which was equipped with P-47 Thunderbolt fighters, and the 6th Night Fighter Squadron which flew Northrop P-61 Black Widows. The Thunderbolts were also responsible for suppressing the Japanese-held Mariana Islands, though it proved difficult to stop the 3,600-man garrison from repairing the airfield on Pagan. In addition, the US Army stationed elements of eight anti-aircraft gun battalions and two searchlight battalions on Saipan between July 1944 and February 1945. The head of the USAAF General Hap Arnold, who also directly commanded the Twentieth Air Force, was concerned about whether these defenses were sufficient, particularly given the high cost of the Superfortress bombers. Accordingly, he allocated a Microwave Early Warning (MEW) radar to protect the bases on Saipan. This advanced radar was not immediately installed, however, as the local commanders believed that the island's air defenses were adequate.