Final answer:
The USSR had significantly weakened Nazi Germany by the time of the D-Day invasion, and while it's possible they could have eventually defeated Germany without the Allies, the joint efforts of the Allies undeniably hastened the end of the war.
Step-by-step explanation:
Considering the historical events and the might of the USSR and Allies during World War II, it has been argued that the Soviet Union played a critical role in defeating Nazi Germany. While the impact of the D-Day invasion on June 6, 1944, cannot be understated in opening up a second front against Nazi Germany, thus relieving some pressure on the USSR's Red Army, it's important to recognize that by then, the Soviet forces had already turned the tide against the Nazis. The Red Army had won significant victories, notably the Battle of Stalingrad, which had already weakened the German army substantially. Moreover, by the time of the D-Day landings, the Soviets were pushing Nazi forces out of the Soviet Union, demonstrating a considerable military might that could argue for the possibility of them defeating Germany without the full scale of Allied help, though this would likely have lengthened the war and potentially resulted in even greater losses for the Soviet Union.
However, the Allied landings at Normandy indeed sped up the process of liberating Europe from Nazi control, with over 150,000 troops landing on the first day, taking the Germans by surprise, and reinforced by several hundreds of thousands more, leading to the eventual liberation of Paris and the drawback of German forces from the Western Front. This massive operation, along with the overall joint efforts of the Allies, significantly contributed to weakening the Nazi regime, making the combined Allied victory possible.