Traffic analysis can reveal a fleet before codes are broken

Reading every Japanese message is unnecessary for the Allied intelligence picture. The timing and extent of code penetration remain uncertain, but existing knowledge could help analysts make faster progress under wartime pressure. Long-range aircraft from the Philippines could also reveal a strike force, while coordination between the Japanese army and navy creates another communications weakness. Traffic analysis can identify where messages are going and changes in their volume without deciphering their content. With warning, British night attacks could damage and track the carriers before an American dawn strike reached them without a broad search. That sequence depends on the intelligence arriving in time to establish the enemy's position.