Answer:
Both sides began stockpiling nuclear weapons.
Americans feared the growing power of the Soviet Union and worried that communism might spread.
What were the causes of the Cold War?
The Marshall Plan.
The Soviet Union held a sizable portion of Eastern Europe on the eve of Germany's capitulation, and the Western Allies took a sizable portion of the west. Both the west and the east were unable to agree on how to divide Europe.
As a consequence, this is what happened at the moment. Parts of the old German territory were given to Poland and the Soviet Union, and Germany was split into four zones of occupation. Due to their betrayal at the Yalta Conference and subsequent Soviet takeover, Poland played a significant role in the early Cold War. (a historical enemy of Poland). One of American President Franklin D. Roosevelt's final decisions was the contentious Treaty of Yalta. Roosevelt justified his actions by pointing out that Poland was occupied by the Soviet Union. The major victory for the west in the meeting was the decision to conduct free elections in Poland after the war. But this pledge would not be fulfilled.
Harry Truman, F.D.R.'s vice president, succeeded him as president shortly after F.D.R. passed away naturally. However, Stalin and the Soviet control that had been created at Yalta were unpopular with Truman. Truman and Churchill both opposed the creation of the Soviet proxy state in Poland. After all, Churchill had hosted the Polish administration in exile. Numerous British officials protested the Yalta choices and the deportation of Polish citizens, which led to a crisis in the British administration. (many of whom would be executed back in Poland). However, it was already too late to change anything at this time. At this time, the Pacific War was still raging.
There was even more contention over the division of Europe, particularly in Germany and how they were to be handled, at the following significant conference in Potsdam (the first of Truman's significant talks with Stalin). At this point, Truman also told Stalin about the atomic device, a brand-new weapon. Two of these atomic weapons would be detonated on Hiroshima and Nagisaki just days after the meeting, and the war would end shortly after with the Japanese capitulation.
Relations between the West and the East were once more strained as a result. Stalin was indignant that the Soviet Union had no sway over continental Japan. After all, they had attacked Manchuria and Korea in accordance with their agreement. (which to their credit, had contributed greatly to the Japanese surrender). Stalin ultimately succeeded in annexing the Kuril Islands and the remainder of Sakhalin, which are still a part of Russia today.
As a result of American concerns that the Soviet Union would seize control of the entire Korean peninsula, an arrangement was made to divide Korea at the 38th Parallel. Despite the fact that this was initially done with the goal of creating a single Korean state, growing unrest elsewhere made this split irreversible.
Marshall Plan
Back in Europe, we have what ultimately cemented the Cold War into place — the Marshall Plan. The plan was a replacement for the original plan on the treatment of Germany called the Morgenthau Plan. The Morgenthau Plan essentially meant to turn Germany into a non-industrial agrarian state. All of the industries were to be dismantled, and it is likely that millions would’ve starved. However, this idea was quickly thrown out in the west, and only partially adopted in the east through the dismantling of industries and infrastructure as reparations.
In 1946, Churchill made his famous “Iron Curtain” speech, which only served to further cement antagonism between the Allies and the Soviet Union. In 1947, President Harry Truman adopted the “Truman Doctrine.”
The Truman Doctrine was the basis for the position of the United States against communism for the rest of the century. Greece was in the middle of a Soviet-sponsored Civil War. The Truman Doctrine allocated 400 million dollars towards aiding the Greek royalists against the communists after Britain had exited the conflict in 1947. The new foreign policy of the United States was known as “containment.”