"Stickleback" completed outfitting on 26 May and held her shakedown cruise off the California coast. She reported to Commander, Submarines, Pacific Fleet, for duty on 21 June. More modern equipment was installed at Pearl Harbor; and, on 2 August, she arrived at Guam, where she held sea trials for a few more days. She began her first war patrol on 6 August when she departed for the Sea of Japan. She arrived there the following week and began patrolling. However, the atomic bombs had been dropped on Hiroshima and Nagasaki and it was believed the war would end shortly.