The plans for the attack fell into enemy hands so the German's knew were the next landing location was. Many Allied troops were killed even before making it to the ground. But maybe more important...
There were probably more than 11,413 Allied caualties and just less than
6,000 Axis Killed in Action. Civilian caualty numbers were low but that is
still disputed.
United State and Britain was the main opponent for Germany. There were some air force from Poland and Netherlands. Canada was involved as well but didn't really do anything, they just want to prove...