The Turnaround:

These early successes of Germany were however short-lived. The surprise turnaround came when in June 1941 Germany forced Russia to side with the Allies by launching a joint attack on Russia. Initially Germany managed to destroy a major part of Russia but by December 1941, Moscow's staunch resistance had stopped the flow of victories for Germany.

Quickly after that the United States was forced to give up its neutral stand and join the war. Since U.S. has planned not to join the war, it was safely watching the progress and was not exactly a threat to any country. However in Dec 1941, Japan made U.S. join the war by attacking Pearl Harbor without any declaration of war. U.S. entered war with a vengeance only to see Germany and Italy declare war on it. Initially there were many setbacks for the Allies in Asia but by summer 1942 things began to change. This happened because Germany had been in the war for more than two years now. Its army was tired, its resources depleted and its artillery and weapons weakened. But at the same time, the biggest force in the Allies, the U.S. had just started using its full force and was in absolute command of the situation.

Germany suffered first major defeat when Britain captured and liberated North Africa in 1942. Just the very next month, America invaded Algeria and free forces of France quickly joined Allies. Allies launched a solid attack and soon...

At the same time, German 6th Army surrendered to massive attacks in Russia and after that, Russia quickly advanced to other areas.
By 1944, it was almost all over. The Allies had been winning consistently since 1942. German forces were completely depleted and also suffered serious setback to their morale. The fateful landing at Normandy by the Allies sealed the fate of German forces.

Foregone Conclusion

It was not exactly a foregone conclusion till the U.S. had not joined the war. As long as it was simply Britain fighting against the Axis, it looked that Germany could easily win at every single battle. However the biggest mistake was made by Japan when it forced the U.S. To join the war and after that, there was simply no looking back for the Allies. Once the U.S. was in the war, it gave a major boost to the Allies and bolstered their confidence. Secondly U.S. had previously been underestimated, it was later found out just how powerful their defenses were. By the time this was realized, the damage had already been done.

References

The Columbia Encyclopedia, Sixth Edition Copyright© 2004, Columbia University Press.

Bradley Lightbody, the Second World War: Ambitions to Nemesis (New York: Routledge, 2004)

Lightbody, p.1

Columbia Encyclopedia, Sixth Edition, 2004

Columbia Encyclopedia