...the Allied forces breached Fortress Europe during World War II. We know now it as "D Day" (there were actually many "D Day"s, that name was not originally specific to Operation Overlord's invasion), but there was a lot of planning that went on way before the day and fighting that occurred after.
Over on my reading list (yes, I have one... yes, I can read), you'll note that I have Stephen Ambrose's D Day: June 6, 1944: The Climatic Battle of World War II. If you haven't read it, I suggest you follow the link through to Amazon and put it in your shopping cart. This book is so good, my wife (who read it first) and I actually vacationed once in Normandy just so we could get a feel for it all (that's also something I recommend if you ever get the chance).