What were the relations/mood between the USA and USSR during WWII, and how did it change later?

There is an old saying that goes, "The enemy of my enemy is my friend." That saying sums up the relationship between the Soviet Union and the United States during World War 2. They each had a common enemy -- Nazi Germany -- and were allied against that enemy during World War 2.

Prior to WW2 relations between the two nations were frigid at best. During the Russian Civil War between the Bolsheviks (Red Russians) and Czarist forces (White Russians) in the aftermath of WW1, the United States landed forces in Serbia in support of the White Russians. After the Bolsheviks came to power, they established the Comintern in 1919. The Comintern was dedicated to spreading the Communist revolution worldwide and included at least four groups from the United States.

Now, one thing that mitigated open diplomatic hostility between the two nations during the interwar years was the United States' withdrawal from international affairs and into isolationism following WW1. Still, the United States did not embrace the Communist government of the Soviet Union.

When we look at WW2, we need to understand that the United States entered the naval war in August 1941 even though it was officially neutral. Starting on Aug. 12, US warships escorted British convoys as far as Iceland and Greenland. US forces also took over the occupation of Iceland following the British invasion of the island. And, in June 1941, the US Congress passed the Lend Lease Act, which allowed the Untied States to lease military equipment to Britain.

Shortly after Germany invaded Russia in June 1941, the terms of the lend lease were also applied to the Soviet Union. And, United States warships helped escort convoys carrying military supplies to Russian ports as well. Again, the cause of this support was not love for the Russian government, but the enemy of my enemy is my friend.

Early in World War 2 there were some hopes that the experience would bring the two nations together. However, by the end of WW2, those hopes were only held by naive idealists. Others in the United States government and military, such as Gen. George S. Patton, suggested rearming the defeated German military and sending it back east into Soviet-occupied Eastern Europe once the Nazis had been defeated. The Soviet Union, for its part, moved quickly to solidify the gains its armies had made during World War 2, establishing Soviet-dominated governments in Eastern Europe and forming what would be known as the Iron Curtain.

Also, American fear of Soviet intentions may have led to a quicker dropping of the atomic bomb on Japan. In April 1941 the Soviet Union signed a five-year non-aggression pact with Japan, which held for the duration of the five years. Following the surrender of Germany in 1945, the Soviet Union declared war on Japan on August 9. Along with concerns that an invasion of Japan would cost up to 1 million American military lives and 5-to-10 million Japanese lives, concerns about expanded Soviet influence in Asia resulted in the United States dropping the atomic bombs on Japan.

So, in short, to describe the United States and the Soviet Union as allies during World War 2 might be a little generous. A better description might be that of co-belligerents against a common enemy, Germany. There were three changes brought about by World War 2 that brought the Soviet Union and the United States into direct competition.

First, Germany was defeated and the common cause shared by the two nations was no longer in existence.

Secondly, European colonial empires, namely those of the Dutch, English and French, were irrevocably weakened. Following World War 2, the United States became the strongest Western power on the earth and so assumed the leadership role that belonged to England prior to World War 2.

Thirdly, the United States shed the isolationism that marked its foreign policy in the interwar years. After World War 1, the United States retreated to its borders behind the Atlantic and Pacific Oceans. After World War 2, the United States continued to take an active role in world affairs. And, this, more than anything else, brought the United States into confrontation with the Soviet Union. Prior to WW2, the US displayed animosity to the Soviet Union but, with a foreign policy dictated by isolationism, that animosity did not lead to the conflict experienced between the two nations following WW2. After WW2, America's direct involvement in world affairs caused the two nations to come into real conflict throughout the world.

Basically no. Hitler formed alliances of convenience. He signed with the Soviets to keep the war on one front. When he thought the war in the west was won, he attacked his allies in the east. You're right about there not being a cold war following WWII because neither the US nor the USSR would have existed had your plan come to fruition. Not all problems are solved with money. If the US had joined Germany, that would have made us accessories of Hitler's "solution", and no better than the man we fought against. You can never know what might have been. The arms race was a natural progression following WWII. Countries found out how easy it was to actually invade and take over another country. You can't take bomb money and give it to the people. It doesn't work that way. There's no way to say where the money that didn't go to bombs might have gone. A better solution might be to tell the people to send a certain amount of money to the government every year. Say you have to pay $10,000 in taxes. Why can't you send it to the government agency of your choice instead of sending it to the IRS? The answer is because if you did, then the politicians wouldn't have a job. Just so you'll know, it wasn't Patton who wanted to fight the Soviets. It was MacArthur who wanted to fight the Chinese after defeating North Korea in 1953. Harry Truman made sure he didn't get his wish.