How were the USSR and England very important to the war in Europe?
If the soviet Union OR England had fallen to Germany, how would that have made the U.S' part in the war more difficult, if not impossible?
From a militaristic and tactical point of view, both nations were extremely important to the Allied victory. It should be noted that when the Soviet Union had forged an alliance with Hitler, the prospects for victory seemed farther off in the distance. Hitler's violation of this alliance allowed the Allied forces to have a presence on the Eastern and the Western side to Hitler, allowing them to strike from both ends. The presence of both sides on the German army allowed for a greater chance of success. In being able to trap the Germans from both sides, it is difficult to envision success being present with one of these sides absent.
Both of these would have made a huge difference.
If Great Britain had fallen, it would have been almost impossible to invade Western Europe. There would not have been this safe haven where the US could bring all its equipment and from which it could bomb Europe. So Western Europe would have been solidly held by Germany.
If the USSR had fallen, so much of Germany's army could have moved over to the west to defend that area and D-Day could probably not have succeeded. The Russians killed more Germans than anyone else -- if they had been out, things would have been way easier for Germany.
In matters of war, there are too many "ifs and buts" involved in conjectures like this to give a definite answer. Some of the main considerations would be the extent to which the resources of Germany would have been sapped in the process of these victories, and their ability to retain the control of new territories in the long run. Then there is the question of future of different countries that include the allied forces, neutral countries and even the Axis forces. There is no reason to assume that there would have remained complete harmony among all Axis force. counties. Then what about impact of some independent developments like the nuclear bomb.