By Carl LaVO, Courtesy MSNBC.com

Today, the nation marks the 68th anniversary of the Allied invasion of the coast of Normandy in France to free Europe from the reign of German dictator Adolph Hitler during World War II.

Since June 6, 1944, the invasion has been referred to as D-Day. But what does “D-Day” stand for? Where did the name come from?

There is little agreement on a definitive answer. However, most historians believe it was a carryover from a military code used in World War I. The Army, at that time, used the term H-Hour to indicate the time of a military action, and the term D-Day for the day it would occur.

The code was used to maintain secrecy about the actual hour and day of major operations.

The D-Day invasion of June 6, 1944 was of epic proportions. An armada of 5,000 vessels transported more than 150,000 men and nearly 30,000 vehicles across the English Channel to France in one operation. Six parachute regiments of more than 13,000 soldiers simultaneously deployed over Normandy from more than 800 planes. An additional 300 planes were used to attack enemy fortifications with 13,000 bombs.

By the time the sun went down on June 6, more than 5,400 Allied soldiers were dead. But 100,000 others had overrun the Nazis and secured landing zones for the successful push toward Paris and Germany to finally topple Hitler’s Third Reich regime.

During the Allied invasion of Normandy, an estimated 6,500 German soldiers perished.