American carrier doctrine revealed what the missing Japanese carriers meant

American exercises had already demonstrated the pattern of an unseen carrier approach followed by a surprise strike. A large concentration of Japanese carriers suggested a major objective rather than a minor target. Pearl Harbor and the Philippines were both plausible destinations in this assessment. Without satellites, American authorities could not simply locate the carriers and settle which target had been chosen. That uncertainty strengthened the case for placing both bases on full alert. Assuming that Japan would not dare attack American territory left a known possibility inadequately covered.