I gather that the Allies were pretty hopeful about the success of D-Day, but if the landings had failed, or if they lost the Battle for Normandy, what were their alternative long-term options? Was there ever a possibility of surrender if Rommel had thrown them back into the sea, like Hitler thought, or was this just a desperate fantasy of his? Would the Allies have been able to attempt another landing somewhere else, or would they have been reduced to bombing Europe from the air while the Soviets rolled Germany up all by themselves?
And as a follow-up question - with the vicious bocage fighting, and failure to make the initial gains they had planned for, how close were the Allies to actually losing in Normandy? Did the Germans ever have a real chance of victory or was it just a matter of time as soon as the beachhead was established?
I'd like to add that during the Normandy invasion, the Allies also landed in Southern France in September - Operation Dragoon. Also the allies were advancing in Italy, they moved north and captured Florence between June and August 1944. Interestingly the Brazilian 1st Infantry Division fought in Northern Italy during WWII.
However there is an answer to this on here already:
https://www.reddit.com/r/AskHistorians/comments/2em57g/what_was_the_back_up_plan_if_dday_failed/
It was just a matter of time. The Germans had always been weak where logistics were concerned and their industrial capacity was severely degraded by strategic bombing. They added whatever running vehicles they could to their motor pool wherever and whenever they could find them, and horses were still in widespread use as prime movers, especially in rear areas. Imagine being a Wehrmacht truck driver six or seven hundred miles into the Russian countryside looking for a new clutch for your broken-down 1932 Citroen bakery van? They couldn't match America and Britain in terms of ability to move men and material or in quality of those men and material. German mechanized equipment was sometimes failure-prone and couldn't be produced in numbers to match what we could. To give you an idea, the Germans produced about 200,000 Opel, MB, and Borgward trucks over the course of the war. The United States turned out about 800,000 trucks in the 2-1/2 ton class alone. Once we had that beachhead and the industrial might of the US to hold it, it really was only a matter of time.