Twelve military historians have contributed to a new book, The D-Day Companion, commemorating the 60th anniversary of the largest and most successful amphibious assault in history.

The world had never seen anything like the D-Day landings. The numbers are staggering. An armada of 7,000 ships sailed across the English Channel toward the coast of Normandy. Overhead, more than 11,000 military aircraft supported the landing and transported paratroopers.

Samuel Newland, a historian at the U.S. Army War College, describes that first day of the invasion. ?This is unbelievable,? he says. ?Sunset on June 6 we had landed something like 155,000 men in the daylight hours. That is almost unbelievable. About 75,000 landed on the beach the first day, along with 23,000 paratroopers. That is a monumental achievement.?

To the German troops defending the beaches, the sight of the awesome armada emerging out of the fog had a huge psychological effect.

Colorado College military historian Dennis Showalter has written extensively about the German military. He tells the story of German fighter pilot Pips Priller, who flew over the Normandy beaches and caught sight of the entire Allied fleet.

?He said you flew across the beaches and it was as though you saw a whole people coming onshore,? Mr. Showalter adds. ?He said I knew then we were finished.?

The Allied troops and their leaders were less certain than the German pilot. D-Day was the result of years of careful planning and rigorous training.

But Carlo D'Este, a retired U.S. Army Lieutenant Colonel who has written several books analyzing World War II battles, says tremendous risks remained, beginning with the weather.

?Just imagine 1944,? he explaines. ?You did not have the sophisticated computer-driven stuff that they have [now]. All you had was weather balloons. You had these things they dropped into the middle of the Atlantic that supposedly fed them information and you had just experience and everything else to try to put together.?

Colonel D'Este is the author of a biography of General Dwight Eisenhower, the commander of the allied forces who was known as Ike and later became president of the United States. The colonel says no one understood the risks of the operation better than the men who planned the Normandy invasion and then had to wait in Britain for news.

?They did not know anything about what was going on on the beaches, and it went on all day long,? he says. ?The phone kept ringing, and Ike was sitting there smoking one cigarette after another, 'What the hell is happening? What the hell is happening?' and they really could not tell him. It was hours before they even had a clue.?

Professor Dennis Showalter says the astounding mass of material, the legendary planning, the bitterly fought battles and the great heroism of D-Day often overshadow the enormous risk involved.

?It was for the western allies the supreme risk because it could not be done a second time, not the same way,? he explaines. ?Had the invasion failed on the beaches, it is very possible that [British Prime Minister Winston] Churchill's government would have collapsed. It is very possible that [U.S. President Franklin] Roosevelt would have lost his bid for a fourth term. It is very likely that the Soviet Union would have been able to extend its power to the Rhine River and perhaps the Atlantic Ocean.?

In the opening chapter of The D-Day Companion, Samuel Newland writes that one of the most important lessons to be learned from the Normandy invasion is the power of a cohesive alliance.

?What we did was a classic example of an alliance that worked together closely and the power that alliance warfare has, if correctly done,? he said. ?There is not really a better example in modern warfare, in my opinion. Maybe that is not new, but for the type of world we are going into in the 21st century, I think it is important to think back about what an alliance can do for you. ?

As memorable as the events of D-Day were, the real lesson, Carlo D'Este says, should be found in the cause, not the magnitude of the battle.

?Something Ike said,? he recalls. ?I think is words really sum it up as well as anything. 'We did not come to gain anything. We came to do what we had to do, restore freedom.'?

Colonel D'Este concludes The D-Day Companion by writing that the spirit and common purpose of D-Day are what we celebrate on June 6.