What were the underlying causes to the outbreak of the Cold War between the USA and the USSR?
There were numerous factors involved in the development of what came to be known as the Cold War.
The United States and Soviet Russia, the country that came into existence after the Bolshevik Revolution of 1917, had very different outlooks regarding how governments should be selected and should function; the United States advocated a democratic government with leaders elected by the general population, with multiple viewpoints represented by the elected officials, while Soviet Russia developed an all-powerful single party system of government with leaders elected by a very small number of officials within the party. The United States followed a capitalist economic policy which supported free market enterprise with minimal government interference; Soviet Russia's communist government controlled production and distribution of goods and services and private ownership was prohibited. The conflicts between these radically opposed perceptions of the world were a starting point.
Following the end of World War I, the United States was able to return to relative isolation, protected by oceans between the North American continent and other countries. Soviet Russia did not have this geographic protection and feared foreign invasion by enemy countries. The feelings of insecurity and potential threat were heightened when Soviet Russia was not included in international diplomatic efforts, largely because other countries didn't want to risk an expansion of the Russian governmental dominance of territory and people.
World War II validated Soviet Russia's fears of invasion from the west as Germany attacked. The United States contributed its military involvement to the Allied forces fighting Germany in western Europe rather than supporting Russia's fight, privately conceding that Soviet Russia would probably end up controlling some Eastern European countries, starting with Poland, after the war.
When President Franklin Roosevelt died and was replaced by President Harry Truman, the United States's Supreme Commander became someone who had not worked with Soviet leader Joseph Stalin during the war and who didn't trust Stalin to abide by agreements that were developed to determine the division of German-conquered territory after the conclusion of World War II. Russia wanted Germany severely punished and left as weak as possible after the war; the United States supported efforts to rebuild a strong Germany. Divisions of Germany and Berlin among the Allied Powers didn't end the disagreements.
Following the end of World War II, relationships between the United States and Soviet Russia degenerated as conflicting economic policies led to clashes regarding the rebuilding of areas shattered by the effects of the war, as implications of the United States's use of the nuclear bomb and Soviet Russia's efforts to develop one became known, and as Soviet Russia supported individuals fighting the government of Iran with the hope of gaining access to Iran's oil resources.
On March 5, 1946, former British Prime Minister Winston Churchill used the phrase "Iron Curtain" for the first time to describe the iron-clad control taking over and enclosing countries that were being brought under the control of Soviet Russia. Churchill urged the United States take a stronger role in opposing further expansion of the Soviet sphere of influence.
Bernard Baruch, an advisor regarding financial and atomic energy affairs to President Truman and others, first used the phrase "Cold War" in 1947.