What was the reason for the breakdown in friendly relations between the United States and the Soviet Union after World War II? What were the results of this conflict?

It is safe to say that the United States and the Soviet Union had an uneasy relationship and alliance from the start. During World War II, both nations were united in their common goal of defeating Nazi Germany (as well as Japan). When that was accomplished, there was little to keep them on good terms.
Soon after the war, disagreements over what to do with the countries liberated from the Germans arose. It was an ideological schism in many ways. The United States, as well as Great Britain, supported the idea of establishing democratic nation-states. The Soviet Union wanted communist countries that would operate within its sphere of influence. It also liked the idea of having these nations serve as a physical buffer between the Soviet Union and the West.
The new nuclear capabilities of the United States also soured relations. The Soviets recognized that even though they had a massive army, the United States' possession of the atomic bomb was a destabilizing element. This led them to steal nuclear secrets from the United States. This, of course, began the nuclear arms race which would define much of the Cold War.

Immediately following World War 2, the United States and the Soviet Union began breaking ties. During the war, they worked together to defeat Nazi Germany and the Axis powers, but they had separate ideals. The Soviet Union, being near the conflict, wanted to prevent their own conquest, while the United States was trying to quell the spread of fascism. When Germany was overthrown, the communist Soviet Union began building their own strength and conquering lands near them.
America, observing the Eastern Hemisphere getting swallowed up by the Soviet Union, grew worried about their power and their disdain for democracy. Additionally, the two nations had divvied up the Nazi scientific leaders, and now Soviet Russia was building stronger missiles and developing nuclear weapons. The United States naturally feared this power and hated the differing ideals, and the two nations fell out completely, engaging in an escalating cold war of weaponry.

During World War II, Great Britain, the US, and the Soviet Union were allies against the Axis Powers. This meant that they had the common goal of stopping Germany, Japan, and Italy from taking over Europe. In February of 1945, Stalin, Roosevelt, and Churchill met at the Yalta Conference to discuss the plan for Germany after the war. Stalin’s presence in Europe outnumbered that of the US and Great Britain, and the fight against Japan was not over yet. Because of this, Roosevelt did not want to push Stalin too hard from his path because the U.S. needed the help from the Soviet Union against its fight with Japan.
In July of 1945, President Truman met with Stalin and the new Prime Minister of Great Britain, Clement Atlee. During that meeting, a plan was made for Germany to be divided into four zones, each being occupied separately by the US, the Soviet Union, France, and Great Britain. Stalin wanted to keep all of the nations he had occupied during the war, making them communist nations as well, whereas the US wanted the Soviet Union control to be temporary. Stalin would not promise to allow free elections in those nations.
Once the war was over, the Allies split Germany into two parts: East Germany, led by communist Stalin, and West Germany, led by the noncommunist United States. Just as Stalin wanted, almost all of the nations in Eastern Europe became communist states under Soviet control.
The Soviet Union and the United States were now superpowers, but now that Germany had been defeated, the two nations had little in common. The US was a capitalist democracy and believed in free elections. The Soviet Union was a dictatorship, meaning that Stalin controlled all aspects of the nation, and those who defied or questioned him could be imprisoned or put to death.
Truman was worried that the spread of communism would not only affect Eastern Europe, but also that it would extend into other parts of the world. To counteract the spread of communism, Truman promised aid to nations struggling against communism. The Soviet Union naturally backed any nation wanting the opposite. Because both nations were so very different, it was only a matter of time before their friendship became shaky.