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Adrian Lewis is a professor of history at Kansas University and author of "Omaha Beach: A Flawed Victory."

When Adrian Lewis used to teach at the U.S. Military Academy at West Point, he took cadets to the beaches of Normandy for three years in a row. There, he tried to explain to them — and understand himself — why the Allied invasion was executed the way it was.

"It defied common sense," Lewis says. "A daylight assault on a deliberate defense, without long preparation?"

Lewis was so intent on answering the question that it became the subject of his dissertation as a Ph.D. candidate at the University of Chicago, which was later turned into a book titled "Omaha Beach: A Flawed Victory," published in 2001. Lewis also appeared in May on the PBS show NOVA, in an episode titled "D-Day's Sunken Secrets."

As the world marks the 70th anniversary of the Normandy invasion's first day, known as D-Day, scholars and military historians are still making sense of the many factors that determined success in the largest seaborne invasion the world has ever seen.

D-Day marked the beginning of an Allied attack on German-occupied Western Europe. The campaign would eventually take pressure off Russian forces fighting Germany on its eastern flank and pave the way for the liberation of France from Nazi forces.

Lewis, now a professor of history at Kansas University, found plenty of reasons why the battle happened the way it did.

For one, the plans were a collaboration among several nations, including the U.S., the United Kingdom, Canada and the French resistance. It was also a collaboration between the U.S. Army and Navy, two military institutions that Lewis points out had no history of working together.

"When you think about the plan and all the players involved, there was a compromise," Lewis said. "It worked, but it was not the best plan, and it almost failed."

Long before landing a single troop on the French coast, the Allies had to choose where and when to attack. Several factors went into the selection of time and place for an invasion, Lewis notes.

The Allied forces, led by famed military commanders such as General Dwight Eisenhower, wanted a spot close enough to England to give air support time over the field to hit targets, yet wanted to avoid the obvious invasion spots. Stable weather was also critical for shuttling in tens of thousands of troops from the seas.

After considering more than 1,000 miles of beaches, Allied generals settled on Normandy. The Allies did everything they could to obscure their plans from German intelligence. Even so, troops ran into strong, if incomplete, German defenses. As a result, Allies suffered heavy casualties and early failures on the invasion's first day.

Lewis said from his own trips to Normandy and those of the cadets he taught, the sheer scale of the battle tends to amaze. Fighting all day long, troops had to cover miles of beach ground — under machine gun fire and over mine fields. "People don't understand the magnitude of this thing," Lewis said.

Had the Allies failed at Normandy, Lewis thinks it would have taken them at least six months to regroup and recharge their forces, giving German commanders an opportunity to bolster the country's eastern front against Russia's forces. That, in turn, could have allowed Germany to negotiate a settlement with the Allies.

But the plan didn't fail. From his own study of the battle for Omaha Beach, a section of the coast fought by the battle-tested 1st Infantry Division of the U.S. Army, Lewis has considered the reasons for Allied success.

"Too often when thinking about war, we concentrate on the generals," he said. "On Omaha Beach, it was soldiers that made the difference."

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