Operation Neptune was part of the larger Operation Overlord, the Allies' undertaking to invade Western Europe and free the nations from the control of Nazi Germany.
After intense and successful deception of the Axis forces, both operations began on Tuesday, June 6, 1944, with the storming of France's Normandy shore.
Alongside the Allied military on D-Day
Alongside the Allied military on D-Day
U.S. troops wade ashore from a Coast Guard landing craft at Omaha Beach during the Normandy D-Day landings near Vierville sur Mer, France, on June 6, 1944 in this handout photo provided by the US National Archives. On June 6, 1944, allied soldiers descended on the beaches of Normandy for D-Day - an operation that turned the tide of the Second World War against the Nazis, marking the beginning of the end of the conflict. REUTERS/Robert F. Sargent/US National Archives/Handout via Reuters
FRANCE - JUNE 01: A Convoy Of American Soldiers In A Military Barge On The Point Of Landing On The French Beaches Of Normandy Between June 6, 1944 And July 15, 1944. (Photo by Keystone-France/Gamma-Keystone via Getty Images)
6th June 1944: American troops come ashore at Omaha Beach in a life-raft after their Landing Craft Vehicle-Personnel had been sunk off the Beachhead. (Photo by Weintraub/MPI/Getty Images)
U.S. reinforcements land on Omaha beach during the Normandy D-Day landings near Vierville sur Mer, France, on June 6, 1944 in this handout photo provided by the US National Archives. On June 6, 1944, allied soldiers descended on the beaches of Normandy for D-Day - an operation that turned the tide of the Second World War against the Nazis, marking the beginning of the end of the conflict. REUTERS/Cpt Herman Wall/US National Archives/Handout via Reuters
World War II, More and more German prisoners are gathered together on Utah Beach after the allied Normandy landings, Around June 6, 1944. (Photo by Photo12/UIG/Getty Images)
Omaha Beach landings, D-Day, the Normandy Invasion, June 6, 1944. (Photo by CORBIS/Corbis via Getty Images)
Hundreds of American paratroopers drop into Normandy, France on or near D-Day, June 6, 1944. Their landing, part of an all-out Allied assault from air and sea, was the beginning of a sweep through Europe that would finally defeat Nazi Germany. (Photo by ï¿½ Hulton-Deutsch Collection/CORBIS/Corbis via Getty Images)
A British ship launching a depth bomb to hit German submarines off the coast of Normandy. Normandy, 6th June 1944 (Photo by Mondadori Portfolio via Getty Images)
A group of U.S. wounded soldiers sheltering behind a wall after the Normandy landing on the beach called Omaha Beach in code. Normandy, 6 June 1944 (Photo by Mondadori Portfolio via Getty Images)
World War II, US soldiers on a Normandy Beach, June 6, 1944 (D)-Day. (Photo by: Photo12/UIG via Getty Images)
FRANCE - JUNE 01: World War II. Normandy landings. American soldiers helped by their companions after the wreck of their boat at their arrival at Utah-Beach (Manche), June 6 1944. (Photo by Roger Viollet/Getty Images)
FRANCE - JUNE 01: Troops And Boats Arriving On A Beach Of Normandy On June 6, 1944, Or In The Days Which Followed. (Photo by Keystone-France/Gamma-Keystone via Getty Images)
American soldiers go ashore during the Normandy landings. landing operations on Tuesday, 6 June 1944 (termed D-Day) of the Allied invasion of Normandy in Operation Overlord during World War II. (Photo by: Universal History Archive/UIG via Getty Images)
FRANCE - JUNE 01: American Troops Landed On The Beaches Of Normandy From June 6 To July 15, 1944, In Order To Liberate France From German Occupation. (Photo by Keystone-France/Gamma-Keystone via Getty Images)
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The weather on D-Day was less than ideal, but the Allies were relying on very specific tides and moon phases, a perfect mix of circumstances which prevented them from postponing the attack.
Roughly 50 miles of the Normandy shore were targeted. The coast was broken up into five sectors, codenamed Omaha, Utah, Juno, Gold and Sword, which were attacked by 156,000 troops led by future President Dwight D. Eisenhower.
Casualties were highest at Omaha beach, and 4,414 Allied soldiers were confirmed dead in total.
Operation Neptune ended as a decisive victory, as the United States, United Kingdom, Canada and the rest of the Allies established five new beachheads on the Normandy shore.
The Normandy landings remain the largest seaborne invasion in history, and many believe the operation signified the beginning of the end of World War II.