Thanks for that good post Carl. I guess I have to explain myself a bit better
IRT Technological aspects, we didn't have the mass or momentum in 1942 that we had in 1944. What I meant was not to speak of necessarily the inferiority of weaponry in direct comparison, but the lack of development in comparison to 1944. We developed our weapons to be particularly efficient at their tasks - based on experience
Additionally, our troops were far less experienced. I don't know if Mortman's around to back me up on this, but it's not just the Generals that win wars, it junior officers and NCOs/Staff NCOs - and their level of experience makes a huge difference. Not to mention the riflemen.
The distance of airfields from the front lines was certainly an issue early on, but as time went on and aircraft missions had to be prioritized, there were significant hiccups in getting CAS where it needed to be within a short enough order - a system we eventually got better at with the advent of TACPs and airborne controllers during the invasion of Italy. The difficulty of air ground coordination was so great that the US had to rewrite its air C2 doctrine almost entirely in FM100-20 (July '43).
The Sledgehammer invasion force would have been a mostly British force, so that does lend itself to somewhat easier coordination.
However, the lack of available landing craft meant that no more than 6 Divisions could land. If the forces were to land in southern France, they might have had a chance to hold out for Roundup but would still have been exposed to a longer drive toward Germany and a large concentration of Axis troops to the south in Italy and in Northern France. Sledgehammer, however called for the invasion across the channel - which in my opinion would not have lasted due to the relatively small size assault force. Not only do we have to put the troops there - we then have to supply and reinforce them - something that might prove difficult, particularly if the Luftwaffe could have organized an aerial interdiction campaign like we used in North Africa - the Afrika Korps and Luftwaffe were eventually all but starved of supplies due to our bombing of their vital supply shipping. Yes you have the fact that aircraft could fly from England, but they'd likely end up facing the same problem the Luftwaffe did over Britain with gas and minimal station time to defend the convoys.
Sources include:
Aerial Interdiction in Three Wars, Mark (pp21-25);
Airpower and Ground Armies - Essay 1: Getting Together, Orange