Operation Sledgehammer was an Allied plan for a limited cross-channel invasion of Europe to take place in 1943. The prime objective was to capture the port of either Cherbourg or Brest to establish a defensible foothold and staging area for an invasion force in northern France. The US favoured this option over Operation Husky (the Allied invasion of Sicily), but the British opposed it since Churchill favoured the soft underbelly strategy into Italy which was the weakest and most vulnerable of the European Axis powers and because there weren't enough landing craft. It was abandoned in favour of Operation Torch and later the Dieppe raid showed the difficulties in capturing a port with determined enemy resistance.
Suppose Roosevelt had pressed through what they wanted and Operation Sledgehammer had taken place somewhere in late 1942 or early 1943 instead of OTL's Operation Torch. What would have happened? I see some immediate problems. First, the balance in air power will be less favourable to the Allies. This way, German panzers in northern France could be able to launch a big counteroffensive and push the Allies back into the sea. Second, this invasion force would be smaller due to less landing craft being available. Third, it relies on capturing a port which Dieppe later showed to be very difficult.
The operation is likely to fail IMHO which could postpone any invasion of north Africa and later Italy for many months. With the balance in the air more equal, the Allies will lose more landing craft, ships and troops. This has potential to be a catastrophe for the Allies (although not an insurmountable one). I'm not sure whether the Allies would try again after such a debacle.
What do you think?