I-20 and the other four "mother" submarines proceeded to the planned recovery area for their midget submarines west of Lanai, where they spent the night of 7–8 December 1941. None of the midget submarines returned. Early on 9 December 1941, I-18, I-20, and I-24 received orders to leave the recovery area. I-20 departed the Hawaiian Islands on 12 December 1941 and arrived at Kwajalein with I-16 on 22 December 1941. First war patrol On 4 January 1942, I-20 departed Kwajalein to begin her first war patrol, assigned a patrol area in the vicinity of Fiji and the Samoan Islands. She surfaced off the harbor at Pago-Pago on Tutuila in American Samoa before dawn on 11 January 1942 and fired twelve rounds from her deck gun at the naval station. Most of her shells missed, and the only casualties were a United States Marine Corps officer and a Samoan Marine Reserve Battalion member who suffered wounds. Moving to Fijian waters, I-20 attacked the Royal New Zealand Navy armed merchant cruiser on 16 January 1942 just after Monowai departed the harbor at Suva. Her torpedoes exploded prematurely at 16:03. Believing that she was attacking a merchant ship, I-20 surfaced from Monowai at 16:08 to attack her with gunfire. Monowai opened a heavy volume of fire with her portside guns. I-20 traded fire with Monowai, claiming a hit on Monowai′s bridge, but submerged at 16:14 after Monowai straddled her. Neither vessel suffered damage, and Monowai transmitted a submarine contact signal and steamed out of the area at high speed. I-20 returned to Kwajalein on 24 January 1942, then proceeded to Japan, where she arrived at Yokosuka on 1 February 1942. February–April 1942 During I-20′s stay in Japan, the German naval staff in Berlin formally requested on 27 March 1942 that Japan begin attacks on Allied convoys in the Indian Ocean. On 8 April 1942, the Japanese formally agreed to meet this request by dispatching submarines to operate off the coast of East Africa, and that day they withdrew Submarine Division 1 of Submarine Squadron 8 from its base at Kwajalein to Japan. By 16 April 1942 they had created the "A" detachment within Submarine Squadron 8, consisting of I-20 and the submarines , I-16, I-18, and , as well as midget submarines and the auxiliary cruisers and , which were to operate as supply ships for the submarines. That morning, the commander of