Operation Overlord

Operation Overlord was the codename for the Battle of Normandy, the Allied operation that launched the successful liberation of German-occupied Western Europe during World War II. The operation was launched on 6 June 1944 (D-Day) with the Normandy landings (Operation Neptune). 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 cross-channel landings in 1944 was made at the Trident Conference in Washington in May 1943. American General Dwight D. Eisenhower was appointed commander of Supreme Headquarters Allied Expeditionary Force, and British General Bernard Montgomery was named commander of the 21st Army Group, which comprised all the land forces involved in the operation. The Normandy coast in northwestern France was chosen as the site of the landings, with the Americans assigned to land at sectors codenamed Utah and Omaha, the British at Sword and Gold, and the Canadians at Juno. To meet the conditions expected on the Normandy beachhead, special technology was developed, including two artificial ports called Mulberry harbours and an array of specialised tanks nicknamed Hobart's Funnies. In the months leading up to the landings, the Allies conducted Operation Bodyguard, a substantial military deception that used electronic and visual misinformation to mislead the Germans as to the date and location of the main Allied landings. Adolf Hitler placed Field Marshal Erwin Rommel in charge of developing fortifications all along Hitler's proclaimed Atlantic Wall in anticipation of landings in France.

The Allies failed to accomplish their objectives for the first day, but gained a tenuous foothold that they gradually expanded when they captured the port at Cherbourg on 26 June and the city of Caen on 21 July. A failed counterattack by German forces in response to Allied advances on 7 August left 50,000 soldiers of the German 7th Army trapped in the Falaise pocket by 19 August. The Allies launched a second invasion from the Mediterranean Sea of southern France (code-named Operation Dragoon) on 15 August, and the Liberation of Paris followed on 25 August. German forces retreated east across the Seine on 30 August 1944, marking the close of Operation Overlord.

In June 1940, Germany's leader Adolf Hitler had triumphed in what he called "the most famous victory in history"—the fall of France. British craft evacuated to England over 338,000 Allied troops trapped along the northern coast of France (including much of the British Expeditionary Force) in the Dunkirk evacuation (27 May to 4 June). British planners reported to Prime Minister Winston Churchill on 4 October that even with the help of other Commonwealth countries and the United States, it would not be possible to regain a foothold in continental Europe in the near future. After the Axis invaded the Soviet Union in June 1941, Soviet leader Joseph Stalin began pressing for a second front in Western Europe. Churchill declined because he felt that even with American help the British did not have adequate forces to do it, and he wished to avoid costly frontal assaults such as those that had occurred at the Somme and Passchendaele in World War I.

Two temporary plans code-named Operation Roundup and Operation Sledgehammer were put forward for 1942–43, but neither was deemed by the British to be practical or likely to succeed. Instead, the Allies expanded their activity in the Mediterranean, launching Operation Torch, an invasion of French North Africa, in November 1942, the Allied invasion of Sicily in July 1943, and Allied invasion of Italy in September. These campaigns provided the troops with valuable experience in amphibious warfare. Those attending the Trident Conference in Washington in May 1943 took the decision to launch a cross-Channel invasion within the next year. Churchill favoured making the main Allied thrust into Germany from the Mediterranean theatre, but the Americans, who were providing the bulk of the men and equipment, over-ruled him. British Lieutenant-General Frederick E. Morgan was appointed Chief of Staff, Supreme Allied Commander (COSSAC), to begin detailed planning.

The initial plans were constrained by the number of landing craft, most of which were already committed in the Mediterranean and in the Pacific. In part because of lessons learned in the Dieppe Raid of 19 August 1942, the Allies decided not to directly assault a heavily defended French seaport in their first landing. The failure at Dieppe also highlighted the need for adequate artillery and air support, particularly close air support, and specialised ships able to travel extremely close to shore. The short operating range of British aircraft such as the Supermarine Spitfire and Hawker Typhoon greatly limited the number of potential landing-sites, as comprehensive air support depended upon having planes overhead for as long as possible. Morgan considered four sites for the landings: Brittany, the Cotentin Peninsula, Normandy, and the Pas-de-Calais. As Brittany and Cotentin are peninsulas, the Germans could have cut off the Allied advance at a relatively narrow isthmus, so these sites were rejected.

The Pas-de-Calais, the closest point in continental Europe to Britain, was the location of launch sites for V-1 and V-2 rockets, then still under development. The Germans regarded it as the most likely initial landing zone and accordingly made it the most heavily fortified region; however, it offered the Allies few opportunities for expansion as the area is bounded by numerous rivers and canals. On the other hand, landings on a broad front in Normandy would permit simultaneous threats against the port of Cherbourg, coastal ports further west in Brittany, and an overland attack towards Paris and eventually into Germany. The Allies therefore chose Normandy as the landing site. The most serious drawback of the Normandy coast – the lack of port facilities – would be overcome through the development and deployment of artificial harbours.

The COSSAC staff planned to begin the invasion on 1 May 1944. The initial draft of the plan was accepted at the Quebec Conference in August 1943. General Dwight D. Eisenhower was appointed commander of Supreme Headquarters Allied Expeditionary Force (SHAEF). General Bernard Montgomery was named commander of the 21st Army Group, which comprised all of the land forces involved in the invasion. On 31 December 1943, Eisenhower and Montgomery first saw the COSSAC plan, which proposed amphibious landings by three divisions, with two more divisions in support. The two generals immediately insisted on expanding the scale of the initial invasion to five divisions, with airborne descents by three additional divisions, to allow operations on a wider front and to speed up the capture of the port at Cherbourg. This significant expansion required the acquisition of additional landing craft, which caused the invasion to be delayed by a month until June 1944. Eventually the Allies committed 39 divisions to the Battle of Normandy: 22 American, 12 British, 3 Canadian, 1 Polish, and 1 French, totalling over a million troops.