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Seventy-three years ago, Americans, British, Canadians, Free French Forces, and their allies launched the most complex operation ever implemented by human beings: The invasion of Normandy.

So much planning, training, coordination, equipment, and courage had to come together for success that it is still hard to believe the Allies achieved it. If you have not seen The Longest Day, it is a good introduction to the complexity of the invasion. And the first 25 minutes of Saving Private Ryan is perhaps the most authentic representation of the horrors of Omaha Beach on film.

Amphibious landings, like the ones conducted by Allies on the beaches of Normandy, are the most complex undertakings in warfare. Normandy was especially difficult because the Germans had had four years to prepare for an invasion. Both the Nazis and the Allies recognized that this was the crisis of the war in Europe.

If the Allies failed, they might never have had the nerve to try it again – and even if they did, it would have taken at least a year, possibly two, to rebuild the forces needed for another invasion. The human cost of failure would have been appalling – with tens of thousands of American and Allied forces losing their lives.

In the end, D-Day was remarkably successful. The three paratrooper divisions landed well behind the beaches and did their job of blocking German reinforcements. All five beaches were captured. By the end of the day, the Allies were ashore in such large numbers that the Germans could not force them back into the sea. The gamble was won and the Allies were on the way to victory 337 days of hard fighting later.

There are four courageous moments that made the D-Day success possible.

First, in January 1944, British General Bernard Montgomery insisted on dramatically increasing the size of the operation to ensure that it was so large that it would probably succeed despite everything the Germans could do to try and stop it. Dwight D. Eisenhower, who was then Supreme Commander of the Allied Expeditionary Force in Europe, agreed, and together they forced the development of a much larger campaign.

Second, Eisenhower had concluded that the Allied strategic bombers had to be used to cut off the French railroads. This would make it much more difficult (and much slower) for the Germans to move forces to the point of invasion. British Prime Minister Winston Churchill, however, was concerned about the number of French civilian casualties that would result from this decision (strategic bombers were very inaccurate in that era) and feared the negative impact this would have on the British-French relationship following the war. General Charles de Gaulle, the leader of the Free French movement, reassured him that France would pay any price to get rid of the Nazi occupiers. Eisenhower warned that they would agree to the bombing campaign or he would resign. He reasoned that he was the person responsible for success, and if they did not trust his judgment, they needed a new commander. Churchill ultimately agreed, and the railroads were bombed, slowing the Germans dramatically in their effort to rush troops to Normandy.

Third, the invasion was originally planned for June 5, 1944, but the weather was so bad that it had to be postponed. Seven thousand ships and more than two hundred thousand troops were at sea. Three divisions of paratroopers and their planes and gliders were leaning forward, ready to go.

The Germans, however, decided to relax. They thought the strong storms, high winds, and dark skies made an invasion impossible.

But the winds were coming from the west, and the Allies, who had a weather station in Greenland, received advanced warning that the weather was about to improve rapidly. The most the meteorologist could promise Eisenhower was a two-day window of fairly decent weather.

At this point, the burden of the biggest decision of World War II fell solely on Ike's shoulders: Wait for better weather and keep all those soldiers, sailors, and airmen waiting – which risked the invasion’s discovery by the Germans – or take the gamble and move forward with the attack in the early hours of June 6th.

Eisenhower said go. He then kept in his pocket a message stating that he took full responsibility if the invasion failed. But Eisenhower was right to gamble on the improving weather. Had he waited until later in June, the invasion would have been shattered by one of the greatest storms to hit Normandy in years.

Finally, the courage of the fighting men cannot be overstated. My wife, Callista, and I have spent the past week visiting Normandy with the City of Fairfax Band, of which she is a member. The band has performed at ceremonies at the Brittany American Cemetery in Saint James, the Normandy American Cemetery at Omaha Beach, and at a commemoration event at Sainte-Mère-Église.

Sainte-Mère-Église was the first place American paratroopers arrived at the very beginning of the invasion. One paratrooper, John Steele, had his parachute caught by the church steeple and hung there for ten hours with the church bells hammering in his ears. Red Buttons played him in The Longest Day. For the observance, the people of Sainte-Mère-Église have a parachute with a model of an American paratrooper hanging from the church. This town goes back to at least 1080, and it is a wonderful combination of past and present. It may be the most pro-American place in all of France.

At Omaha Beach, we confronted the raw courage of the Americans who faced an almost hopeless task. As S.L.A. Marshall recounted in a superb article for The Atlantic in 1960, “only six rifle companies were relatively effective as units,” because “three times that number were shattered or foundered before they could start to fight.” The deep courage of the men who were watching a massacre and forced themselves onto the beach and then up the hill against entrenched and well sited German machine guns and mortars should inspire all of us.

In the end, courage is necessary for freedom to survive. There was a poignancy of touring Normandy while terrorists were killing people in London. As I cited in remarks at the cemeteries, evil exists.

In World War II, evil was embodied in Hitler and the Nazis. Today, evil is embodied in the terrorists who kill civilians and seek to impose their religion on the rest of us.

It took courage to defeat evil on D-Day in 1944. It will take courage to defeat evil today.