With the Russians rolling in towards Nazi Germany from the Eastern Front, Britain and the US needed to get a move on.
Initially reluctant to thrust his forces into the hornets' nest in Normandy, Winston Churchill came to realise that relying on the Russians to do most of the fighting perhaps wasn't such a good idea after all.
After the Battle of Stalingrad, the pendulum of the war had begun to swing the other way. The Nazi war machine, triumphant and aggressive at first, was now being driven back to Germany.
This was good in the short term, but it could have proved disastrous for post-war Europe. The more territory gobbled up by Stalin, the greater the number of countries that would go from fascist to communist rule when hostilities ended.
That meant Britain, the US and other allies would need to invade northern France, and that would not be easy.
Churchill had conceived of it as the hard snout of the Nazi crocodile. Up until mid-1944 the plan had been to nibble at the softer underbelly of this crocodile first. This is why American and British-led Allied operations in the early part of World War Two had been focused around North Africa and Italy.
But at some point, the focus would have to shift, and that point was June 6, 1944.
The popular image of what followed, made truly iconic by the film 'Saving Private Ryan', is heroic British, American and Canadian troops slogging up Normandy beaches to drive the enemy back to Berlin.
This image is accurate, but it doesn't tell the whole story.
What D-Day really consisted of is both the beach landings that allowed the enormous invasion force to get ashore and the vast web of interconnected elements that allowed this to happen.
The first link in this complex chain was the 1940 evacuation of British forces at Dunkirk, because once the Germans had conquered this corner of the continent it would need to be retaken for the war to be won.
Another important link came on June 14, 1943. This marked the authorisation of Operation Pointblank, a joint RAF Bomber Command/US Eighth Air Force mission to begin destroying aircraft factories on the continent. Over time the steadily-depleted Luftwaffe would be less able to harass the invasion of France, when it finally came.
Before it did, years of research would go into planning it.
Photo reconnaissance pilot Raymond Beckley knew full-well how important his job was. Speaking to the Military History network, he said:
"Generals have to make decisions based on several factors: Are the beaches [a] practical place for landing? What are the problems [going to be] when you get to the beaches? How heavily are they defended?"
Nearly a million reconnaissance pictures were assembled the in the years before the invasion and painstakingly pored over to answer these questions.
When they had been, the gears of the giant Allied military machine started to move towards invasion, but not before a great deception.
Huge numbers of fake tanks and other military equipment were used to help stage a military façade and give Hitler the impression that attack on France would be launched nearer to Calais.
The real hammer blow would, of course, land 250 miles to the west.
When it did, the first soldiers to land in France were men of the Oxford and Bucks Light Infantry.
They didn't do so on the beaches though. They entered enemy territory under the cover of darkness at 12:01 am on June 6, towed in three gliders.
Their mission was to capture Pegasus Bridge, which spanned the Caen Canal. Allied forces would require it to get across the canal, as they drove inland after hitting the beaches.
To call these men brave would be an understatement. Gliders were flimsy, relatively slow, and, if spotted, vulnerable to anti-aircraft fire. Unlike paratroopers, men aboard gliders were not issued parachutes, so if their glider was hit, there was no chance of escape – every one of them would tumble to their deaths. Many fully expected to die that day.
As it turned out, they weren't spotted and their craft hit the ground as planned, right near Pegasus Bridge. They sprung out and dashed for the canal. They wanted to get away from their wooden planes, which could have turned into death traps if they'd come under fire. Also, more crucially, they'd been warned that the Germans might bomb the bridge if they thought it could fall into Allied hands.
The first shots of D-Day were fired by Lieutenant Den Brotheridge as he rushed out of the darkness, his pistol cracking.
His men came up behind, their weapons also soon blazing, and the defenders were quickly driven off. Unfortunately for the Oxford and Bucks men this did not happen before they'd lost two men. One was Lance Corporal Fred Greenhalgh, who drowned in a pond after his glider landed. The other was Brotheridge, who became the first man to die during D-Day when he was cut down by German machine-gun fire on the bridge.
Still, the fight was not over. One veteran, Denis Edwards, remembered:
"What really frightened us was the 21 Panzer [Division], which was a real battle-hardened group in the area around Caen, six or seven miles from where we were landing. And they told us they had around 350 tanks between them, and we thought 'My God, they only need half a dozen of those to come up the road and we're in trouble.'"
Within a few hours, two tanks from 21 Panzer had come up in support.
The men on the bridge prepared their two PIAT anti-tank weapons. They were pessimistic, according to John Tillet:
"They [PIATs] were terribly inaccurate weapons. They were difficult to fire and they were only effective out to about a maximum of about 400 yards, [but more likely] probably [only] 50."
The men got around these limitations by putting a few men on the other bank of the canal and lying in wait until the first Panzer got close by. What happened next may have been, according to Military History, the most important shot of D-Day.
When it came, the German tank that was hit burst into flames and blocked the road. Soon afterwards another PIAT shot hit the one behind. The bridge was saved.
Denis Edwards recalled the scene:
"I think that caused pandemonium for the Germans. The guys in that tank obviously didn't survive. One of them had his legs blown off and he was laying on the road, screaming his head off. The other tank, when that one was hit, blew up, [and] went into fast reverse and trundled off the way they'd come. I think they probably reported back that we has all sorts of anti-tank equipment on the bridge and it wasn't safe for them to go near the place."
While the Oxford and Bucks soldiers had been waiting nervously for the tanks on Pegasus Bridge, they would have heard thousands of aircraft in the skies above them.
This was the next element of the attack. 23,400 American and British paratroopers were about to be dropped at strategic points across Normandy so that they could take out artillery guns and keep routes out of the beaches open.
One of these paratroopers was Lieutenant Dick Winters, of 'Band of Brothers' fame. He was a member of the 101st Airborne Division, who, along with the 82 Airborne, and their counterparts in the British 6 Airborne Division, knew how things were meant to go:
"When it comes around to the time to jump, the planes are supposed to slow down to about 95 to 100 miles an hour. But in this case, when we started hitting anti-aircraft fire and couple of planes started getting hit, many pilots starting taking evasive action. My plane was hit [and] went into a dive. At the same time he [the pilot] went into a dive he threw me the green light to jump."
Winters leapt out and parachuted to earth as the plane crashed behind him. He, and his comrades, had been sprinkled randomly all over Normandy with many of their weapons and equipment ripped off them during the high-speed drop.
It would take hours for them to orient themselves on the ground and find their objectives, but when they did, the anti-aircraft guns were silenced and strongpoints secured.
The next planes that came over the Channel would be bombers. Hitler had augmented the natural defences of the Normandy coast with the 'Atlantic Wall', a subterranean fortress on reinforced concrete with retractable artillery guns on rails, preceded by obstacles to frustrate the advance of infantry and vehicles up the beaches.
It was at this point that luck also came to the aid of the Allies.
The Germans had agonised over how to array their forces, with one school of thought lobbying for men placed close to each beach area so they would be ready to pounce during any attack. Another argument was that a centralised reserve pool should be formed some way behind the beaches, then moved to wherever was most necessary.
In the end, a compromise between these two was reached, with some level of resistance present across the coast and a less centralised reserve held further back. These men were not to be deployed anywhere without authorisation from the top. The trouble was, Hitler was in Germany and not on the ground, able to see what needed to be done. Likewise, Erwin Rommel, one of Hitler's best generals and the man in charge of the Atlantic Wall, was also absent. Mrs Rommel's 50 birthday was on June 6 and he had gone to be with her.
The result of this was hesitation on the part of the Germans, such as the 21st Panzer Division, which did not send more tanks to Pegasus Bridge.
The next domino to fall in the operation came in the form of 5,112 bomber aircraft that flew into to support the invasion. They released their bombs over the beaches to destroy as much of the defences as they could.
Somebody who witnessed this part of the operation was B-17 gunner Frank Maize:
"We just carpet-bombed a strip from out in the water, inland, for the invading forces to come in on. I saw shells exploding in the water, on the land, on the shore and I was glad that I was not down there."
Unfortunately, bombing at Omaha Beach seems to have been rather ineffective because many of the defences there were left in place.
The first men ashore were members of Dog, Easy and Fox Companies of the 2 Ranger Battalion – Rangers were commandos modelled on their British Royal Marine and Army counterparts.
These men were tasked with scaling Pointe du Hoc, a 100-foot cliff overlooking Utah and Omaha beaches thought to be host to six 155mm enemy artillery guns.
Scaling the cliffs would not be easy, but Ranger James Eikner remembers feeling up to it:
"We were ready for action, we were gung-ho fellas, we loved excitement and adventure. And of course the most adventurous activity you can get into is warfare - slightly dangerous now and then, but it's exciting."
Unfortunately for the Rangers, they first ended up in the wrong place and the commanding officer had to direct the landing craft to change course. This 38-minute delay gave the Germans ample warning that they were coming, and they greeted the men of 2 Rangers with grenades and rifle fire as they commenced scaling the cliffs.
Several of the Americans were killed, but most made it to the top. When they did, they were greeted by a First World War-like lunar landscape, pitted with craters from the bombing runs that had come in before them.
Skirmishes ensued as the commandos leapfrogged from bomb crater to bomb crater, exchanging fire with the German defenders.
When they reached the bunkers they were aiming for, they quickly realised that the 155mm guns had been moved so that they would not be captured. Tell-tale tracks had been scratched on the roads and the Rangers followed them one and a half miles inland to an apple orchard where they'd been concealed.
Unguarded, the weapons were quickly dispatched with incendiary grenades. They also blocked the roadway to German reinforcements, leaving the way more open for their comrades about to sweep ashore.
It took two days for them to be reinforced, and when they finally were, on June 8, only 90 men from the original 225 remained alive and unwounded.
Their 2 Rangers comrades in Able, Baker and Charlie Companies would come ashore at Omaha Beach along with men in the 1 and 29 Infantry Divisions.
Artillery and mortars were fired at them as they sped in aboard the landing craft, unnerving many a man.
For their part, the German defenders, veterans of fighting with Russia in the east, were also intimidated. Franz Gockel of 716 Infantry Division recalled:
"When the landing fleet arrived, it was unbelievably huge. We said, 'this can't be happening - so many ships! The horizon was black with ships. And when they came closer, we quickly got into the bunkers and manned the machine-guns."
And man them they did. Omaha turned out to be the most heavily defended beaches of D-Day and the battle that raged there was particularly bloody, with the men of the 2 Rangers and other units in the 29 Infantry Division coming under savage fire.
One veteran from the 29 Division, Robert Sales, recalled what it was like to exit the landing craft:
"When that ramp went down the captain went off first. By the time he hit the end of the runway he was full of bullets."
Sales was the only one to get out of his boat alive. He and his comrades were caught in the crossfire between two machine-guns in the cliffs above the beach.
The action at Omaha is graphically portrayed in Saving Private Ryan - men blown apart and picked off by machine-guns and snipers as they tried desperately to establish a foothold on the beach. Eventually, they did, but not before losing several thousand men.
To the east, on Gold, Juno and Sword Beaches, the British and Canadians were faring better, though the going was not easy.
Kenneth Oakley of the Royal Naval Commandos was there:
"We went to ground about 10 yards up the beach because the fire was so intense, and I did not perceive it was safe to advance any further."
Here though men were assisted by 21 DD tanks. This moniker was short for 'Donald Duck tanks' because they floated on the water. Oakley said of the tank:
"The gun from the DD tank was absolutely essential. It dominated the beach once it was [ashore]."
Machine-gun and mortar fire was soon swept away, leaving the path open for the British troops.
Though that didn't come without some persuasion. When British soldiers came ashore, Oakley recalled that their first instinct was to make tea on the beaches after their sea journey. It took some persuasion to get them to move the activity inland and out of the way.
This now marked the transition point between Operation Neptune, the amphibious mission to get men ashore, and Operation Overlord, which would be the battle for Normandy.
The former was a naval assignment, and beyond ferrying the men to the beaches, ships also bombarded the coasts to give them support. Sometimes this involved considerable risk to man and machine, as Dean Hatton, aboard the destroyer the USS McCook, recalled:
"The skipper said 'If we have to run this ship ashore to knock out those guns, we're going to do it'. We wanted to protect the troops that were landing because they were taking a horrible beating… We came in within 500 yards, almost scraping bottom for a destroyer. The landing craft were being fired upon and we tried to knock out the gun emplacements before the landing craft went in."
Ultimately, the skilful and dedicated all-arms coordination and support paid off. Despite fierce resistance, particularly at Omaha Beach, the Allies were able to get ashore and capture Normandy. From there it would be another eight months before the war in Europe was won.
For more on D-Day, watch 'Operation Overlord & Neptune' from the Military History channel, or read Osprey Publishing's four-volume account, 'D-Day 1944 (1)', 'D-Day 1944 (2)', 'D-Day 1944 (3)' and 'D-Day 1944 (4)', and for more on the Rangers' assault on Pointe du Hoc, see 'Rangers Lead the Way Pointe-du-Hoc D-Day 1944'. More more military history, visit Osprey Publishing's website.