Most of the events that led to the Cold War happened between the years 1945-1949, but the tensions leading to the Cold War started during World War I. Russia fell out of the first world war with the advent of the Bolshevik Revolution in 1917. The Soviet Union emerged from the revolution using Communism as its key financial and social philosophy rather than Capitalism. Most of the western world was wary of communism, but it was a new philosophy and the world had no idea what it would entail. England, the United States, and France “kept their distance” and chose to observe the new economic system. Stalin agreed to work with Hitler in 1939, but switched sides after Hitler launched Operation Barbarossa. Stalin sided with the United States, England, and eventually France until the end of the war.
Things started falling apart after the Yalta and Potsdam conferences (1945). These two conferences were set up to organize the world after the war. Yalta was held prior to Germany's surrender; however, it focused considerable effort in determining what would be done when that eventually happened. The Yalta Conference favored Stalin, because Roosevelt wanted help from the Soviet Union in the Pacific Theater. Upon returning home, however, Churchill wrote Roosevelt stating, "The Soviet Union has become a danger to the free world."
The Potsdam Conference was held five months later in July. Germany had just surrendered in May. There were a few concerns going into the conference. First, in March of 1945, Stalin had invited the non-communist leaders of Poland to meet with him. Stalin arrested them all. Second, the death of Roosevelt had put Truman in charge. Truman wanted to use a firmer hand with the Soviet Union. Third, during the Potsdam Conference the United States successfully tested the Atomic Bomb. This new power lessened Truman's need to have Stalin help with Japan. Truman's motives during Potsdam changed. He was no longer interested in the Soviet Union entering the war and gaining more power than it already had.
Potsdam did not go as smoothly as Yalta. Germany was broken into four zones, Nazi criminals were to be brought to trial, and Poland was to set up the new government as soon as possible. There were disagreements: a) the details of the boundaries of the four zones, b) the size of the reparations Germany was required to pay, and c) the amount of influence the Soviet Union would have over the Eastern European countries. The Soviet Union's immediate steps to create puppet communist governments in Eastern Europe completely ignored the agreement to allow democratic elections. To the West, this encroachment seemed to be the setup for World War III. Truman saw what was happening in Eastern Europe as almost the same tactics Hitler used in the 1930s. He believed the proper response to the efforts of a bully was to use a credible threat of force, which the United States now had with its ownership of the atomic bomb.
Setting aside "Appeasement," Truman started a new strategy called "Containment." This policy said that the United States would prevent the Soviet Union, militarily if necessary, from using force to expand their communist ideas abroad. The Truman Doctrine would become the cornerstone of United States policy throughout the Cold War. The Truman Doctrine was made official in 1947 when Greece and Turkey, under the threat of Soviet imperialism, was given a gift of financial aid from the US Congress. The money was channeled into the Greek and Turkish militaries in order to thwart Soviet advancement. The United States made it clear that it would do the same for other countries threatened by the Soviet Union. Truman's speech on March 12, 1947, is considered the tipping point where the Cold War started between the United States and the Soviet Union. This doctrine is directly connected to the Korean Conflict and eventually the Korean War.
Under the Truman Doctrine, the United States implemented the Marshall Plan. Under the Marshall Plan, the United States spent billions of dollars to help Western Europe recover from World War II. The United States felt that countries such as France, Italy, and Germany might fall to communism if their economies did not improve quickly.
In 1948, the Truman Doctrine was tested in Berlin. Following the Marshall Plan, the United States, England, and France began implementing efforts for economic recovery in the three German zones that they controlled. This included a new currency system that blocked Soviet Union influence. This currency was to become the legal tender in the Western areas of Berlin. This combining of the three zones, "worried" the Soviet Union. On March 20, 1948, the Soviet Union closed off all western access to Berlin. The Western Allies responded by mounting an airlift dropped 5,000 tons of supplies per day.
In 1949, the North Atlantic Treaty Organization was founded. NATO was a military alliance designed to address Soviet Union imperialism and the spread of communism throughout the world. This along with the Berlin Blockade signaled to the Soviet Union that they were at odds with much of the Western World, commonly referred to as the Cold War. The Berlin Blockade was lifted, but the Cold War would remain for the next fifty years.