I wrote an answer to a very similar question here, that you might be interested in. Because I'm lazy, I'll cut and paste it instead of typing a new reply out.

The importance of the United States to the Allied war effort was absolutely immense, bordering on "absolute necessity" territory. There is a general consensus among military historians that the US's ability to produce war materiel alone tipped the battle in favor of the Allies. Simply put, the US was able to produce war materiel (everything from bullets to trucks to tanks to airplanes to ships to ball-bearings to boots) faster than the Germans (or the Japanese, for that matter) could destroy it or sink it, giving the Allies, by the time of the Normandy invasion, overwhelming superiority of arms on land, at sea, and in the air. And Allied leaders knew this at the time and its importance for the allied war effort. As Winston Churchill noted already in June 1940 (less than a month after the German invasion of Western Europe), winning the war against the Nazis was merely a matter of waiting it out until the Americans stepped in: "We shall never surrender, and even if, which I do not for a moment believe, this Island [i.e. Great Britain] or a large part of it were subjugated and starving, then our Empire beyond the seas, armed and guarded by the British Fleet, would carry on the struggle, until, in God's good time, the New World [i.e. the United States], with all its power and might, steps forth to the rescue and the liberation of the old." ("We shall fight on the beaches" speech). And Stalin famously stated in 1943 at the Tehran conference that "Without American production the United Nations could never have won the war." Note that this statement was made a full 18 months before the German surrender, so that tells you the confidence Stalin had in the Allied superiority of arms.

So to an extent, one could argue, as you did (OP in original post), that, "the U.S. involvement was mainly supplying Allied troops through lending/leasing of fuel and materiel and supplying troops to the Normandy landings." But doing so would be a moderately unfair characterization of the importance of that effort to winning the war. It is not an overstatement to say that the war went from a probable Allied victory in November 1941 to a there-is-no-doubt-about-it Allied victory on Dec. 8, 1941. There was simply no likely military scenario in which the Germans would have been able to defeat an Allied coalition that included the United States.

The biggest problem with the basic outline that I've described above regards the role of the Soviet Union, which is, basically, absent. This is also the major source of misunderstanding as regards how Americans are taught about the war - the role of the Soviet Union is not emphasized to the extent it probably should be. There is absolutely no doubt that the Soviet Union did the vast majority of the fighting (and dying) in support of the Allied war effort. More Allied soldiers died at the Battle of Stalingrad than did on the American side during the entire war (including the Pacific theatre). If American war materiel was decisive in winning the war, then Soviet blood must also be given equal billing. But even without American aid, the longer the war against the Soviet Union went on, the more and more likely it became that the Germans would lose. While benefiting from American aid greatly, and not really measuring up favorably against its American counterpart, Soviet industry had already begun to rebound by 1943/44 and would have quickly reached a point where it could produce war materiel faster than the Germans could destroy it. Even without American aid, an ultimate Soviet victory was, I would argue, inevitable - a matter of when, not if. The real question is whether Stalin, who was notoriously paranoid, could have held out long enough without any outside aid - that's a question for someone who knows more about Soviet history than I do.

You should also check out the replies in that thread, as some give great additional details.