 En route to Japan , the Fifth Carrier Division was diverted to Truk to support Operation Mo ( the planned capture of Port Moresby in New Guinea ) . While they were preparing for the mission , the Americans intercepted and decrypted Japanese naval messages discussing the operation and dispatched the carriers Yorktown and Lexington to stop the invasion . The Japanese opened Operation Mo by occupying Tulagi , in the Solomon Islands , on 3 May . American land @-@ based aircraft had spotted the light carrier Shōhō escorting the transports of the main invasion force on 6 May , and the American carriers moved west to place themselves in a position to attack it the following morning . 
