worried that the Soviets would take their campaign of subversion a stage further and set out to undermine the liberal democracies of Western Europe. This would represent a serious threat to the United States, both economically and strategically.

Most senior policy-makers in Washington agreed with George Kennan that the Soviet threat must be contained, and this inevitably meant that the United States would need to adopt an aggressive posture towards the USSR and the expansion of its sphere of influence.

The Cold War shaped much of world history over the next four
decades after the end of World War II. The Allies argued over the postwar settlement. The United States, Britain, and Russia met at the Teheran Conference in 1944. The decision for an invasion of France left Russia free to move into eastern Europe. The three met again at Yalta in 1945. The Soviet Union agreed to join against Japan in return for territorial gains in China and Japan. The United Nations was confirmed. Agreement over Europe's future was difficult. A disarmed Germany, purged of Nazi influence, was divided into four occupied zones. Eastern Europe, although promises were made for a democratic future, was left under Soviet domination.

The final postwar conference was at Potsdam in 1945. By then, the Soviets occupied eastern Europe and eastern Germany. They
annexed eastern Poland while the Poles gained compensation by receiving part of eastern Germany. Germany and Austria were divided and occupied. Japan was occupied by the United States and stripped of its wartime gains. Korea was freed, but was divided into United States and Soviet occupation zones. Asian colonies returned to their former rulers. China regained most of its territory, but civil strife continued between Communists and Nationalists.

In other regions, colonial holdings were confirmed. In Europe, Russia's frontiers were pushed westward to regain World War I losses. Most nations existing in 1918 were restored, although the Baltic states once again became Russian provinces. All except Greece and Yugoslavia fell under Soviet domination. Western nations were free, but under American influence.

The US and the Soviet Union became Cold War adversaries because they did not trust one another.  Each thought that the other side was going to try to take over the world.  For example, the US thought that the Soviets had taken Eastern Europe and put it behind an "iron curtain," thus going back on the promise of elections that it had made at Yalta.  The communists, meanwhile, felt that American policies like the Truman Doctrine and the Marshall Plan were meant to threaten them.  As they came to distrust one another more and more, they became enemies in the Cold War.

During the Second World War, the United States and the Soviet Union were allies, though scarcely friends, as their cultural and political differences always made the two governments suspicious of one another. At the Yalta Conference after the war, President Roosevelt agreed with Stalin that the Soviet Union should regain control of the defeated or liberated countries of Eastern Europe which were its neighbors. While Roosevelt imagined that this would mean the liberation of the countries in question, Stalin chose to occupy the countries and install Communist systems of government within them, leading to the “iron curtain” which separated Eastern and Western Europe for decades.

In the atmosphere of mistrust created by the failure of the Yalta agreement and the installation of the iron curtain, both huge power blocs discovered that the other had been spying on them, even when they were supposed to be allies. This put the seal on their animosity and ushered in the era of the Cold War.

There were many reasons for this, actually.  The first reason dates back to WWI and the immediate fallout from the Russian Revolution.  America wanted Russia to stay in the war against the Germans after WWI and for a time, the United States sent aid to the republican government in Russia so that it might keep fighting; however, the war became so unpopular in Russia that the communists had no problems convincing the soldiers to rebel and help their cause.  The United States sent troops to the Russian civil war to guard supplies that might be used by the White Army, the side that was czarist and favored a restoration of the Romanov dynasty.  This marked the beginning of Soviet-American distrust.  The White Army ultimately failed, and America pulled its troops out, but would not extend full diplomatic recognition to the Soviet Union until 1933. After WWI, it appeared as though Communism was spreading in Europe, as leftist governments attempted to spread in Germany and France.  There was even a growing Communist movement in America.  The American government thought all of this was controlled by the Kremlin, and this was partially true.  America thought that the Soviet Union sought world domination by creating revolutions in other countries.  During WWII, a truce existed between America and the Soviet Union as they had a mutual enemy in Hitler, but then the Soviet Union accused America of improper dealing as it would not share the secrets of the atomic bomb and Stalin said that the Western allies procrastinated in opening a Western front against the Nazis in order to further drain the Soviet Union of resources and men.  After the war, the Soviet Union occupied Eastern Europe and set up puppet governments there.  When America complained, the Soviet Union said that the United States did the exact same thing in Latin America under the guise of the Monroe Doctrine.  When the Soviet Union unexpectedly developed the atomic bomb in 1949 and American reconnaissance found this out, the U.S. government accused the Soviet Union of having an extensive spy network and started to further its own espionage efforts--another way that Soviet-American distrust grew during the Cold War.    

Another reason that America and the Soviet Union were enemies were ideological reasons.  From the late 1800s, capitalist interests did not like anything that looked like Communism in America.  They tried to get rid of attempts to have unions and after the spread of Communism in Europe after WWI, led the efforts to limit immigration from Eastern Europe in order to try to stop the spread of Communism's ideas here in America. Another reason Americans were against Communism was its stance on religion.  The Soviets under Lenin and even more so under Stalin killed priests and other members of the clergy and destroyed or re-purposed houses of worship.  Stalin even proclaimed that the Soviet Union was the first atheistic state in world history, even though many Orthodox and Jewish groups continued to exist in underground sects.  It was not until 1954 that America added "one nation under God" to its Pledge of Allegiance in an attempt to further distance itself from its Soviet rival.  Remember, also, that the U.S. observes a separation of church and state under its Constitution.