As people remember the 60th anniversary of D-Day, BBC News Online offers a brief reminder of what the campaign was all about.
Up to 3,000 Allied troops died on D-Day alone
What was the significance of D-Day?
D-Day signalled the start of the Allies' invasion of western Europe in June 1944, and a crucial turning point in the war with Nazi Germany.
Russia was making progress towards Berlin, farther to the east.
But Allied commanders agreed a second front was needed to defeat Germans occupying much of western Europe.
Thousands of mainly Americans and Canadians joined British naval, air and ground troops in southern England, to prepare for Operation Overlord.
When did it take place?
D-Day had been planned for more than a year, and those who were to take part spent several months training.
The ambitious air and sea assault was dependent on a combination of factors, including the weather, tidal conditions and most important of all, surprise.
Despite forecasts of poor weather, it was originally scheduled for 5 June.
But storms forced Supreme Allied Commander Gen Dwight Eisenhower to put it back 24 hours. Finally, the weather improved and he gave the command.
How many troops were involved?
A total of 156,000 men took part in D-Day, but many times that number were to be involved in the ensuing campaign over the next few months.
Airborne troops were parachuted into Normandy in the hours before the main seaborne invasion party landed.
A total of 6,000 ships and landing craft were involved, delivering troops to five beaches along a carefully selected stretch of the Normandy coast.
On D-Day alone, up to 3,000 Allied troops died. Some 9,000 were wounded or missing.
What happened on D-Day?
A brilliant deception plan led German military leaders to suspect the main invasion would be farther up the coast.
The surprise element helped British and Canadian troops in particular establish footholds at beaches codenamed Gold, Juno and Sword.
American soldiers also managed to land on the westernmost beach - Utah - without major casualties.
But at nearby Omaha, they suffered severe losses as they encountered a crack division of German troops.
What happened after D-Day?
Once the beaches were secure, progress through the narrow lanes and staunchly defended towns of Normandy was slow.
But with the Allies outnumbering their enemy and supported by their air superiority, they were able to overcome the considerable resistance - though at a heavy price.
By the time they crossed the Seine and liberated Paris in late August, around 10% of the Allies' two million troops had been killed, wounded or were missing.
But the success of Operation Overlord was to pave the way for Allied victory.