In mid-1944, the Allies had already cleared the Axis out of Africa and made significant advances in Italy. On top of this, Soviet forces were making fantastic gains on the Eastern front, well-assisted by Allied lend-lease and intelligence. As far as I've read on the Internet, the Red Army was virtually unstoppable by the Nazis at this point in the war.
Why, then, were two massive, costly amphibious invasions (Operations Overlord and Dragoon) undertaken on the Western front? Could the Allies not have simply rushed for Germany through the rest of Italy or, presumably better, allowed the Soviets to simply defeat Germany using Allied supply and diversion of Germany in Italy?
Two reasons the Allies didn't just let the USSR win. First, one of the key arrangements agreed to in the Casablanca conference in 1943 was to open and maintain a second front against the Nazis. This was at the request of Stalin, as he'd been desperate for a second front since the Germans invaded. Secondly, and perhaps more importantly as far as the Allies were concerned, was to prevent the Red Army from "liberating" all of Europe as they did the countries that became the Eastern Bloc. A secondary strategic aim for the end of the war was preventing Soviet hegemony in Europe. It was even a secondary strategic aim in the use of the atomic bombs in Japan, as the Soviets were moving to invade the home islands from the Northwest.