Editing Battle of Midway (section)

===U.S. code-breaking===
Nimitz had one critical advantage: U.S. cryptanalysts had partially broken the Japanese Navy's [[JN-25b]] code.<ref name="MS-134">{{Harvnb|Smith|2000|p=134}}</ref> Since early 1942, the U.S. had been decoding messages stating that there would soon be an operation at objective "AF." It was initially not known where "AF" was, but [[Commander (United States)|Commander]] [[Joseph Rochefort]] and his team at [[Station HYPO]] were able to confirm that it was Midway: [[Captain (United States O-6)|Captain]] [[Wilfred Holmes]] devised a ruse of telling the base at Midway (by secure [[undersea communications cable]]) to broadcast an [[Plaintext|uncoded]] radio message stating that Midway's water purification system had broken down.<ref>{{cite web |url=http://www.nps.gov/nr/twhp/wwwlps/lessons/90midway/90facts1.htm |title=U.S. National Park Service: The Battle of Midway: Turning the Tide in the Pacific 1. Out of Obscurity |archive-url=https://web.archive.org/web/20150308182159/https://www.nps.gov/nr/twhp/wwwlps/lessons/90midway/90facts1.htm |archive-date=8 March 2015}}</ref> Within 24 hours, the code breakers picked up a Japanese message that "AF was short on water."<ref name="National Security Agency and the Central Security Service">{{cite web |title=The Battle of Midway: How Cryptology enabled the United States to turn the tide in the Pacific War |first=Patrick D. |last=Weadon |date=3 May 2016 |url=https://www.nsa.gov/about/cryptologic-heritage/historical-figures-publications/publications/wwii/battle-midway.shtml |website=NSA.gov |access-date=13 August 2024 |archive-date=18 September 2016 |archive-url=https://web.archive.org/web/20160918013823/https://www.nsa.gov/about/cryptologic-heritage/historical-figures-publications/publications/wwii/battle-midway.shtml}}</ref> No Japanese radio operators who intercepted the message seemed concerned that the Americans were broadcasting uncoded that a major naval installation close to the Japanese was having a water shortage, which Japanese intelligence might have suspected as deception.{{r|baker20160108}} HYPO was also able to determine the date of the attack as either 4 or 5 June, and to provide Nimitz with a complete IJN [[order of battle]].<ref name="MS-138-141">{{Harvnb|Smith|2000|pp=138–141}}</ref>

Japan had a new codebook, but its introduction had been delayed, enabling HYPO to read messages for several crucial days; the new code, which took several days to be cracked, came into use on 24 May, but the important breaks had already been made.<ref name="Willmott 1983 304">{{Harvnb|Willmott|1983|p=304}}</ref> As a result, the Americans entered the battle with a good picture of where, when, and in what strength the Japanese would appear. Nimitz knew that the Japanese had negated their numerical advantage by dividing their ships into four separate task groups, so widely separated that they were essentially unable to support each other.<ref>{{Harvnb|Parshall|Tully|2005|p=409}}</ref> This dispersal resulted in few fast ships being available to escort the Carrier Striking Force, thus reducing the number of [[Anti-aircraft warfare|anti-aircraft guns]] protecting the carriers. Nimitz calculated that the aircraft on his three carriers, plus those on Midway Island, gave the U.S. rough parity with Yamamoto's four carriers, mainly because American carrier air groups were larger than Japanese ones. The Japanese, by contrast, remained largely unaware of their opponent's true strength and dispositions even after the battle began.<ref name="Willmott 1983 304" />