 Complete the following phrase:  In May, Shōkaku and Zuikaku were assigned to a mission to counterattack the American offensive in the Aleutian Islands, but this operation was cancelled after the Allied victory on Attu on 29 May 1943. The sisters were transferred to Truk in July. In response to the carrier raid on Tarawa on 18 September, the carriers and much of the fleet sortied for Eniwetok to search for the American forces before they returned to Truk on 23 September, having failed to locate them. The Japanese had intercepted some American radio traffic that suggested another attack on Wake Island, and on 17 October, Shōkaku and Zuikaku and the bulk of the 1st Fleet sailed for Eniwetok to be in a position to intercept any such attack, but no attack occurred and the fleet returned to Truk. At the beginning of November, the bulk of their air groups were transferred to Rabaul to bolster the defenses there, just in time to help defend the port against the 