The end of the US/Soviet alliance was caused by the radical realignment of world power in 1945 which left two major global power blocs: a European/Anglophone one dominated by the US, and a communist one dominated by the USSR. Their unlikely wartime partnership had transpired, after all, not through shared values or even overlapping geopolitical interests but rather solely due to a common enemy in the form of Hitler. The defeat of Nazi Germany not only removed the only foundation of their alliance but it left both sides in dramatically stronger military & geographic positions relative to before the war.

WW2 inflicted enormous damage on the Soviet Union, but it also provided an opportunity to become a world power. This aligned perfectly with Stalin’s own ambition as well as the desires of many, both inside and outside of the Soviet Union, for the USSR to serve as communism’s muscular standard-bearer on the world stage. As the tide turned against Hitler after the end of 1942, the Soviets forces fought their way westward toward Berlin and evicted German forces from the territory they had conquered. This territory included both Soviet land like Ukraine, but also that of other states like Poland & Hungary.

Once the Soviets arrived in formerly independent states, however, they tended not to leave. Countries which the Red Army entered in 1944-1945 such as Poland, Hungary, Romania, and Czechoslovakia were not restored to their pre-war sovereignty but instead made into Soviet client states. The same occurred in Korea as well, which the Soviets rolled into during their brief campaign against Japan near the war’s end. It was clear to the United States and its partners even before WW2 had been won that the Soviets were exploiting the opportunity to create their own empire. Complementing this was the existence of communist insurrectionary activity in places like China who were not expected to shelve their political ambitions after the defeat of Germany & Japan.

The American power trajectory from 1941 to 1945 was not totally unlike that of the Soviets. In only four years the US went from being a hemispheric power to a truly global one, with America’s newly generated massive military power now abutting the Soviets’ own forces on two continents. While the US-led bloc did not turn its conquered nations into puppets as nakedly & unambiguously as the Soviets did, those states that US forces entered from 1943 onward all remained well within the American orbit and some hosted tens of thousands of US troops (some of which are still there.)

As WW2 unfolded and it became increasingly clear that the combined power of the Allies would eventually crush the Axis powers, it also became increasingly apparent that the alliance between the US/UK-dominated bloc and the Soviet bloc would not long outlive the war. These concerns were very real, but tended to be pushed aside since the more urgent priority was beating Hitler & Japan and the leaders of the Western Allies- FDR & Churchill- rightly realized that harmony between their nations and the USSR was imperative to achieve victory. The implications of Allied victory and the post-war global situation remained, however, an elephant in the room. Though FDR and Churchill have both been heavily criticized for being too trusting of Stalin during the war (i.e. “betraying” Poland and generally failing to anticipate his long-term plans for Eastern Europe), the east/west rivalry that arose almost before the ruins of Berlin had stopped smoking surprised few political or military leaders on either side of what Churchill would soon dub the “Iron Curtain”.

Most of the major participants of WW2 ended the war far weaker than they began. The former Axis powers lay in ruins and even the Allied nations of Western Europe, formerly great empires, began a collective decline in global influence. The only two players who emerged from this brawl more politically & militarily powerful than before were America & the Soviet Union...two guys who would never have teamed up in the first place if they hadn’t both been jumped by the same gang. Hitler inadvertently set the prerequisites for the US & Soviet rises to global power, but he was also the only thing preventing them from becoming rivals.