In March 1942, the U.S. first noticed mention of the "MO" operation in intercepted messages. On 5 April, the U.S. intercepted an IJN message directing a carrier and other large warships to proceed to Inoue's area of operations. On 13 April, the British deciphered an IJN message informing Inoue that the Fifth Carrier Division, consisting of the fleet carriers and , was en route to his command from Formosa via the main IJN base at Truk. The British passed the message to the U.S., along with their conclusion that Port Moresby was the likely target of "MO".