FollowingthePearlHarbordisaster,AmericawonderedwhereandwhenJapanwouldstrikeagain.In1942,anintelligenceunitpickedupanunusualvolumeofcodedradiotrafficfromtheJapanesefleet.Japanwasevidentlypullingtogetherahugefleetforanassaultonatargetcalled"AF".TheletterswerebelievedtostandforMidway.Inordertomakesure,theAmericanssentacodedmessagetoMidwaytellingtheAmericanstoreturnabogusmessage,uncoded,thattheisland'swaterdistillationsystemhadbrokendown.AfewdayslatertheJapanesecodereportedthat"AF"wasshortonwater.
Withthisinformation,theAmericansrushedtopreparethemselves.WhentheJapaneselaunchedtheirattack,theyweredumbfoundedbyacounterattackthatsankfouroftheiraircraftcarriersandendedtheirabilitytotaketheoffensive.
珍珠港一劫之後，美國摸不清日本再度出擊的時間和目標。一九四二年，有情報組人員收到日本艦隊的密碼電報驟增。日本顯然正在組織龐大艦隊，攻擊目標名為AF。這兩個字母，料是代表中途島。為了求證，美軍以密碼電報叫島上美國人回一封欺詐電報，不用密碼，說島上蒸餾水系統壞了。幾天之後，日本密碼電報說AF缺水。
美軍根據所得情報，連忙部署，日本果然出擊，不料遇到猛烈反攻，一時驚惶失措，四艘航空母艦遭重創沉沒，從此再也無力展開攻勢。