On 6 June 1944, more than 156,000 Allied troops landed on the shores of Normandy, in Nazi-occupied France.
The event went down in history as D-Day and it marked the beginning of Operation Neptune – the largest amphibious invasion in history.
Op Neptune, which continued until Allied forces crossed the River Seine on 19 August 1944, was part of the broader operation put in place to liberate German-occupied Europe.
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The wider mission was codenamed 'Operation Overlord' and its success laid the foundations of the Allied victory in the Second World War.
UK personnel from across all branches of the Armed Forces were involved, as were personnel from Allied nations and Britain's overseas territories.
What happened on D-Day?
At 6:30 in the morning (known as H-Hour) on D-Day, American, British and Canadian troops started landing along a 50-mile stretch of coast in Normandy after crossing the English Channel.
The Normandy landings took place on five beaches: Utah, Omaha, Gold, Sword and Juno.
British and Canadian forces faced relatively light German opposition as they captured Gold, Juno and Sword beaches.
Similarly, US troops encountered and overcame light opposition on Utah beach.
However, most of the casualties happened on Omaha beach, where American forces faced heavy combat and resistance and it is estimated that some 2,000 US troops perished.
Despite the German resistance, within less than a week Allied troops had officially conquered and secured the beaches.
Operation Neptune involved nearly 7,000 vessels (80% British) and more than 11,500 Allied aircraft.
D-Day also marked the beginning of the Battle of Normandy, which saw Allied troops fight against the Axis powers (Germany, Italy, and Japan) until 25 August 1944, the day Paris was liberated.
Watch: D-Day veterans recall their experiences.
Why was it called D-Day?
Contrary to popular belief 'D-Day' is not a unique name for the campaign in Normandy.
It is actually a general term for the start date of any military operation used by the United States – the 'D' standing for 'day'.
As Operation Overlord was under the command of US General Dwight D Eisenhower, the American naming convention for that reason was most likely used.
The terms 'D-Day' and 'H-Hour' are used for the day and hour specifically when the day and hour have not yet been determined, or where secrecy is essential.
For all units participating there is only one D-Day and one H-Hour in any given operation.
When used in combination with figures and plus or minus signs, these terms indicate the length of time before or after the planned action.
Thus, the day before 6 June 1944, was known as D-1 and the days after were D+1, D+2, etc.
Why did the Allied Forces choose Normandy as their geographic point of invasion?
Four sites were considered for the landings: Brittany, the Cotentin Peninsula, Normandy, and Pas de Calais.
As Brittany and Cotentin are peninsulas, landing there ran the risk of Germans cutting off the Allies' advances at narrow points.
The Pas de Calais is the closest point in continental Europe to Britain and was as a result seen as the most obvious landing point by the Germans and was therefore heavily fortified.
The other issue was it left little room for further expansion bounded by numerous rivers and canals, unlike Normandy's broad front which allowed Allies to threaten coastal ports further west at the same time.
It also allowed for easier planning of an overland attack towards Paris and eventually into Germany.
Where were the Allied troops from?
Most of the troops that landed on the Normandy shores on D-Day were from the United States (73,000), the United Kingdom (61,715) and Canada (21,400).
However, troops from several other countries were involved in the Battle of Normandy.
That includes personnel from Australia, Belgium, Czechoslovakia, Denmark, France, Greece, the Netherlands, New Zealand, Norway, and Poland.
How many people died on D-Day?
Because of the nature of the fighting, the location of the landings and the impossibility to keep accurate records given the circumstances, to this day it is still not known exactly how many people were killed on D-Day.
Often, it is estimated that about 2,500 Allied Forces lost their lives on D-Day, but more recent research has shown the figure may be closer to 4,500.
German losses on D-Day are equally hard to pin down, with numbers ranging between 4,000 and 9,000 (including killed in action and deaths as a result of wounds).
What factors contributed to the success of D-Day?
Planning for Operation Overlord took nearly a year.
As soon as British Prime Minister Winston Churchill, US President Franklin D Roosevelt and Soviet Premier Joseph Stalin agreed that the first step to winning the war was to create a strategic entry point to Europe via Normandy, plans were put into practice.
Operation Bodyguard, a complex decoy plan, was instrumental in the success of Operation Neptune.
Inflatable hardware, as part of Operation Fortitude, was a key element of Bodyguard and the plan to fool Nazi Germany.
The operation distracted Germany, shifting the attention from the 50-mile stretch of Normandy coast to other parts of Nazi-occupied territory.
Adolf Hitler had expected a large-scale Allied invasion of France since 1942, but the Nazi attention to the French coastline was focused on the main ports, like Calais.
Thanks to an intricate network of deception operations, spies and support from the underground French Resistance, Germany was caught underprepared for an attack on the Normandy shores on 6 June 1944.
Secret informants infiltrated in France provided the Allies with intelligence ahead of the landings and slowed down German defences with acts of sabotage.
What happened after D-Day?
D-Day is referred to by some as the beginning of the end of the Second World War.
After Allied forces landed on the beaches in Normandy, they effectively had a gateway into Nazi-occupied France.
By 11 June 1944, less than a week after D-Day, the five beaches were fully secured.
More than 325,000 troops, 50,000 vehicles, and 100,000 tonnes of equipment had managed to land in Normandy.
In less than two months, by late August 1944, northern France had been liberated.
D-Day was also a significant psychological blow to Nazi Germany.