 On 1 January , the Japanese military changed their radio communication codes , making it more difficult for Allied intelligence , which had heretofore partially broken Japanese radio ciphers , to divine Japanese intentions and movement . As January progressed , Allied reconnaissance and radio traffic analysis noted the buildup of ships and aircraft at Truk , Rabaul , and the Shortland Islands . Allied analysts determined that the increased radio traffic in the Marshalls was a deception meant to divert attention from an operation about to take place in either New Guinea or the Solomons . Allied intelligence personnel , however , misinterpreted the nature of the operation . On 26 January , the Allied Pacific Command 's intelligence section informed Allied forces in the Pacific that the Japanese were preparing for a new offensive , called Ke , in either the Solomons or New Guinea . 
