In November 1943, Joseph Stalin, Winston Churchill and Franklin D. Roosevelt met together in Teheran, Iran, to discuss military strategy and postwar Europe. Ever since the Soviet Union had entered the war, Stalin had been demanding that the Allies open a second front in Europe. Churchill and Roosevelt were hesitant because they believed any attempt to land troops in Western Europe would result in heavy casualties. Stalin, however, didn't trust his allies, and feared they might sign a peace agreement with Adolf Hitler and, perhaps, turn their attention to crushing the communist system in the Soviet Union. Stalin also knew that it would be difficult for his troops to defeat the Germans without the United States fighting them in Europe.
At Teheran, Stalin reminded Churchill and Roosevelt they had promised an invasion in 1942, and again in the spring of 1943, but he saw no sign of an allied invasion of France. By the end of the meeting, the three leaders agreed the Allies would mount a major offensive in the spring of 1944. General Dwight Eisenhower was put in charge of what became known as Operation Overlord.
The invasion plan was completed in the spring of 1944. The American First Army was to land on Normandy in the area of Carentan-Isigny. The Fourth Infantry Division was assigned to invade on Utah Beach, and half the First and 29th Infantry Divisions were tasked with securing Omaha Beach. The British assigned the 50th Division to Gold Beach, and the Canadian Third Division was sent to Juno Beach.
All the land forces were to travel across the English Channel in transports and then board amphibious landing craft that would take them onto the beaches in waves. The troops were to be supported by a naval bombardment against German defensive fortifications and an umbrella of fighter planes to repel any Luftwaffe challenges.
A flotilla of 6,000 ships would transport and back up the operation. The first wave scheduled to hit Omaha and Utah beaches on D day was composed of 60,000 men and 6,800 vehicles for each location. In the next two days, another 43,500 troops and 6,000 vehicles were scheduled to reach Normandy. Similar numbers of British/Canadian troops were assigned the other landing areas. All together, nearly 3 million men in 47 divisions were deployed for the invasion. Most divisions, 21, were American; the rest were primarily British and Canadian, with some French, Polish, Belgian, Italian, and Czech soldiers thrown in. Air support would be provided by 5,000 fighters.
The Allies also coordinated with the French Resistance, but did not give them too much information for fear that the organization might be compromised by traitors. The resistance helped disrupt German lines of communication and sabotaged vital bridges and railways.
In early May, Eisenhower decided that June 5 would be D day, however, poor weather conditions the day before forced him to postpone the invasion until the 6th. Some ships had already left port and had to be recalled. The elements were crucial in planning the campaign. Another delay would have meant putting the whole operation off until at least the 19th, the next date when the moon and tides would be optimal.
The Germans were also watching the weather and felt confident no invasion could take place in early June because high seas would make naval operations difficult, if not impossible, and poor visibility would keep aircraft grounded. Von Rundstedt believed the Allies needed four consecutive days of good weather, and his forecasters said this would not happen in early June. Because the Allies could read the German message in which von Rundstedt made this prediction, Eisenhower knew that he could pull off a surprise if he gave the invasion order at the beginning of the month.