American's did not think highly of the Soviet Union after the revolution, as did most countries, because they refused to recognize the new regime, arguing that it was not a democratically elected or representative government. This was important because it showed that the United States didn't care for the Soviet Union at the beginning of World War 2.
The long term non-regonition policy ended in 1933 when the US, under President Franklin D. Roosevelt's instruction, established full diplomatic relations with the Soviet Union, the last major power to do so.
Further caused alarm in Washington, DC, and made the US hesitate even more with their relations with USSR.
This paved the way for Hitler’s invasion of Poland in September, and it made America hesitate more with their idea and viewing of the USSR.
The United States sent enormous quantities of war materiel to the Soviet Union, which was critical in helping the Soviets withstand the Nazi onslaught. Their relationship began to grow because of this defeat of pushing the Nazi's out of the Soviet Union.
This led to changes in American attitudes. The United States began to see the Soviet Union as an embattled country being overrun by fascist forces, and this attitude was further reinforced in the aftermath of the Japanese attack on Pearl Harbor in December.
These coordinated military actions came about as the result of intensive and prolonged diplomatic negotiations between the Allied leaders, Roosevelt, Churchill and Stalin, who became known as the “Big Three". This united the US and USSR who both had to same ending goal, no matter what they thought of one another, and that was to defeat Hitler and his Nazi militia.