On Thursday, millions of people will direct their eyes to ceremonies honoring the military operation that radically changed and greatly hastened the end of World War II - 75 years after it happened.

On June 6, the U.S. and our allies launched the allied invasion of Normandy known as D-Day. On that date, some 150,000 personnel attacked Nazi-occupied France from the skies, from the sea, and rushed ashore on five strategic beaches and neighboring cliffs to help free a continent from tyranny.

Today, only a tiny fraction of those veterans remain, but the magnitude of what they and their brothers in arms accomplished that day will forever reverberate through history.

So as Americans and our allies commemorate D-Day, what should we remember?  Dr Thomas Conner teaches military history at Hillsdale College in Michigan and is author of "War and Remembrance: The Story of the American Battle Monuments Commission."  He says it's hard to overstate the importance of taking the beaches and establishing another front in the European theater.

"It had to be successful for the Allies to get a foothold on the continent of Europe and begin driving the Germans back in on their own borders.  The war ended in literally the ashes of Berlin, only eleven months after D-Day," said Conner.

Listen to the full podcast to hear Conner explain how the Allies pulled off such a massive mission while keeping the Nazis completely unprepared for it, how close U.S. commanders were to declaring the mission a failure, how American soldiers adapted to parachuting into the wrong places and facing much stiffer German defenses than expected on bloody Omaha Beach and elsewhere along the coast.

He also describes the powerful experience of visiting the U.S. cemetery above Omaha Beach.

"I've seen more grown men cry in Normandy than anyplace else I've ever been and the cemetery evokes that kind of response," said Conner.  "The combination of the memorial but also the awareness that it is right in the center of the Omaha Beach battlefield is quite moving."