page_title: Operation Overlord
section_title: Reconnaissance
hierarchical_section_title: Operation Overlord / Preparations for D-Day / Reconnaissance
caption_attribution_description: English: Map of the air plan for the Allied landings in Normandy on 6 June 1944
context_page_description: Operation Overlord was the codename for the Battle of Normandy, the Allied operation that launched the successful invasion of German-occupied Western Europe during World War II. The operation was launched on 6 June 1944 with the Normandy landings. A 1,200-plane airborne assault preceded an amphibious assault involving more than 5,000 vessels. Nearly 160,000 troops crossed the English Channel on 6 June, and more than two million Allied troops were in France by the end of August.
  The decision to undertake a cross-channel invasion in 1944 was taken at the Trident Conference in Washington in May 1943. General Dwight D. Eisenhower was appointed commander of Supreme Headquarters Allied Expeditionary Force, and General Bernard Montgomery was named as commander of the 21st Army Group, which comprised all the land forces involved in the invasion. The coast of Normandy of northwestern France was chosen as the site of the invasion, with the Americans assigned to land at sectors codenamed Utah and Omaha, the British at Sword and Gold, and the Canadians at Juno.
context_section_description: The Allied Expeditionary Air Force undertook over 3,200 photo-reconnaissance sorties from April 1944 until the start of the invasion. Photos of the coastline were taken at extremely low altitude to show the invaders the terrain, obstacles on the beach, and defensive structures such as bunkers and gun emplacements. To avoid alerting the Germans as to the location of the invasion, this work had to be undertaken over the entire European coastline. Inland terrain, bridges, troop emplacements, and buildings were also photographed, in many cases from several angles, to give the Allies as much information as possible. Members of Combined Operations Pilotage Parties clandestinely prepared detailed harbour maps, including depth soundings.
  An appeal for holiday pictures and postcards of Europe announced on the BBC produced over ten million items, some of which proved useful. Information collected by the French resistance helped provide details on Axis troop movements and on construction techniques used by the Germans for bunkers and other defensive installations.
  Many German radio messages were encoded using the Enigma machine and other enciphering techniques and the codes were changed frequently. A team of code breakers stationed at Bletchley Park worked to break codes as quickly as possible to provide advance information on German plans and troop movements. British military intelligence code-named this information Ultra intelligence as it could only be provided to the top level of commanders. The Enigma code used by Field Marshal Gerd von Rundstedt, Oberbefehlshaber West (Supreme Commander West; OB West), commander of the Western Front, was broken by the end of March. German intelligence changed the Enigma codes right after the Allied landings of 6 June but by 17 June the Allies were again consistently able to read them.
caption: Air plan for the Allied landing in Normandy
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