Donald brings up a good point, and that is how would Stalin have reacted? He wanted the Western Allies to open a second front on the continent at the earliest opportunity as the Soviets had been bearing the brunt of the war against the Wehrmacht. No doubt he would have supported a cross-Channel invasion in 1943 – but how would he have reacted if it had failed to significantly relieve pressure on the Eastern Front? (Of course, the operations in Sicily and Italy didn't significantly relieve pressure on the Eastern Front either, but there was still the hope of an invasion of Northwest Europe that would).Remember, the Allied landings in Sicily rougly coincided with the battle of Kursk. By this time the Soviets had stopped the Germans outside of Moscow, destroyed the German 6th Army at Stalingrad, and pounded the German 9th Army at Rzhev-Vyazma. The Soviets were facing 180 some-odd German divisions while the Anglo-American forces were facing only a handful. If the cross-Channel invasion in 1943 had failed (or had bogged down like it did at Anzio) then post-war Europe would have been significantly different.
Given that in 1943 the Channel defenses were almost non-existent and France was used as a rest and refit post for units battered in the East. I would say that the chances of a successful cross-Channel attack were at least as good as they were in 1944. The fact is that the US did not really want to invade mainland Europe for fear of the casualties they woudl take. If I were a cynic, I would almost say they wanted to wait to let the Russians bleed the Germans some more to make an invasion that much easier when they did finally get around to mounting one.My basic argument is that if they had wanted to, the capability of mounting an invasion of France existed in 1943. The Allies chose not to do so in hopes of mounting a more successful attack in 1944. It was not a matter of can't but won't. The resources and troops were there in 1943. The only fly in that ointment is that any troop replacement crunch post invasion would have been much worse in 1943 than it turned out to be in 1944 and that was bad enough.
Patick – I'd agree up to a point. The British definitely wanted to avoid a cross-Channel invasion as long as feasible, both out of fear of debilitating casualties and in order to let the Soviets grind down the Wehrmacht. But just about everything I have read had the American joint chiefs either arguing against operations in the Mediterranean theater in favor of a cross-Channel invasion or shifting to a de facto Pacific First strategy. It wasn't until the Trident conference in Washington (May 1943) that the British chiefs acquiesced to setting a rough date for the cross-Channel invasion (May 1944) – and that was primarily because George C. Marshall had instructed his staff to threaten shifting to a “Pacific First” strategy if the Brits had stood firm against it.Nonetheless, you raise an excellent point about combat replacements -- U.S. mobilization was only coming into full swing in early '44, pumping out "basically trained" divisions.