why the germans didnt stop the allies at the beaches ? was the luftwaffe finished by that time ?
im trying to understand why the D Day landings went so well and how the allied gain so much territory in the beggining of the campaign .
Firstly its worth noting that the allies did not take much territory initially - the beachhead was as narrow as 8 miles at points until the breakout 6 weeks later though they did take the contentin peninsula by cutting off its base during those weeks. As other posters mentioned they did it with overwhelming force - they had air supremacy and also the coast allowed massive fire support from the fleet including 15 inch guns whilst the fighting was within 15 miles or so of the coast.
The beach defences were never going to hold a large force of this nature for long - the Germans had had to fortify hundreds of miles due to the uncertain nature of the attack point. The plan was always to use German mobile forces to push off any attack but it had to happen within 24-48 hours before too much forces got ashore.
As German buckeye mentioned the German panzer divisions were spread out and came very late to the attack for a number of reasons. Operation mincemeat using a double agent convinced the Germans that Normandy was a diversion for the real attack at Calais - The divisions there remained in place for weeks waiting for this. On the day nearby units were late moving as only Hitler could give permission for them to move and he was in bed till midday and his staff refused to wake him. The bad weather that almost cancelled the operation also meant that the Germans thought it wouldnt happen then - Rommel had gone home to Germany for his wifes birthday. The only German panzer division (at Caen that did respond) set off late in the day firstly went the wrong side of the river Orne (until they discovered that commandos held the Pegasus bridge - drove back through Caen but when they did attack they broke through the coast splitting the beach head. However they panicked and withdrew.
After that it was too late.
So essentially massive and thorough preparation in both forces but also intelligence and deception combined with poor German command structure and some strokes of luck on the day made it happen the way it did. Plus of course that the Germans army and airforce and infrastructure as a whole had been depleted bythe Soviets and allied bombing and unlike the allies they had neither the manpower of the industrial capacity to replace the losses