Few dates of the 20th century matter more. June 6, 1944, found the Allies making their long-awaited charge onto the European continent. If their plan had failed, the history of the Third Reich may have unfolded in a very different manner.

D-Day might be the most famous single day in military history. Everyone knew it was coming, but only the Allies knew when and where they'd strike back against the empire of the Third Reich.

D-Day featured the biggest amphibious landings in human history. Altogether, about 156,000 Allied troops gathered themselves for a charge onto the European continent.

Operation Neptune was the code name for the D-Day landings themselves. This landing operation was the culmination of years of Allied planning.

The Allied invasion of Nazi-occupied France was part of Operation Overlord, which started that June and lasted until August 1944. Overlord was a linchpin of Allied success in the European Theater.

There was no Idaho beach on D-Day. The five iconic code named beaches were Utah, Omaha, Sword, Juno and Gold.

In the hours before the beach landings, Allied paratroopers dropped into areas just inland of the beaches. Their goal was to secure areas for invading troops as they moved from beaches to the inland

Strong currents pushed the ships carrying the U.S. 4th Infantry more than 1 mile from their original destination. But it worked in their favor -- the beach where they landed was lightly guarded, giving them a much easier task than on the other beaches.

U.S. troops of the 29th and 1st Divisions landed at Omaha Beach. They had the misfortune of wading into the worst of the German defenses that day, and incurred the highest casualties of the five landing areas.

Bombers and navy ships pounded German troops at Omaha Beach … but to no avail. The ineffective bombs meant that heavy-duty German fortifications were still intact and ready to repel invaders.

German commanders planned their defenses around the beachhead. They wanted to destroy all Allied units before they could even begin to move inland.

Barrage balloons were sent aloft, and they had long steel cables as tethers. Their purpose? To make it harder for enemy planes to zoom around landing craft and troops below.

The men who survived the initial assault had to deal with relentless enemy machine guns … and tall cliffs. Those cliffs made it very slow going, and exposed many Allied troops to German bullets.

Ten of thousands of Higgins boats were made by Higgins Industries during the war. These boats lowered a bow-mounted ramp, which troops used to run down into the water and onto the beaches.

Higgins boats had enough capacity for a platoon, or about 36 troops. Some of the landing craft sank even before reaching the beaches.

High winds and low clouds meant that Allied ships and planes wouldn't be able to attack on June 5. Thus, the landings were pushed back a day, and still, the weather wasn't optimal.

British troops, along with those from the Netherlands and Poland, were tasked with capturing Gold Beach. They were ordered to secure the beachhead and then rendezvous with U.S. troops who were told to take Omaha Beach.

German commanders figured Allied troops would attack at high tide to minimize troop exposure on the beaches. So they placed obstacles like barbed wire and booby traps at the high water mark to make it much harder for troops to storm ashore.

Allied troops waded ashore, soaked with seawater and under fire from the Germans. Many of them had to run or stumble 200 yards to reach the relative safety of the tall cliffs near the beach. Many of them didn't make it.

The Army wanted a high-tide landing in order minimize the amount of ground infantry had to cover on the beaches. But German defenses were optimized for high tide. Instead, the Allies landed at low tide, when most beach obstacles were visible.

For some troops, it felt like an eternity. The rough waves that day meant that many troops were suffering terrible seasickness after the 17-hour journey from English ports to Normandy. Many of them were so nauseous they were happy to face German troops rather than the ocean.

About 700 Allied warships assisted the beach landings. Thousands of other small ships and landing craft were critical to Operation Neptune's success.

Numerically, the Allies had far greater air power during D-Day, with more than 9,500 warplanes in the vicinity. German forces had only around 800.

Between the high waves and enemy guns, the tanks bound for Omaha Beach were doomed. Only five of the 32 tanks reached the shores.

Allied bombers did their best to soften German defenses at Omaha Beach, but dense low clouds made it impossible to find their targets. Thus, German guns were intact and ready to counter the initial assault.

In April 1944, less than two months before D-Day, Allied troops practiced their beach landing skills with Operation Tiger. But German U-boats interrupted with an attack. Around 750 Allied troops were killed, and the incident wasn't reported in order to protect the secrecy of D-Day.

The epic failure that was Operation Tiger put Operation Neptune in serious jeopardy. Officials nearly called off D-Day for fear that one of Tiger's missing sailors may have been captured, thus endangering the secrecy of D-Day.

The weather wasn't great on D-Day, with churning seas and high winds that morning. But the Allies decided to risk it, and in doing so, they caught the Germans off guard.

At Omaha Beach alone, 55 Higgins landing craft were destroyed in battle, taking with them many American lives. Altogether around 80 of the specially designed boats were lost at the time of D-Day.

Around 4,400 men were killed storming the beaches of Normandy. It was one of the bloodiest days of the war for the Allies.