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The United States and the Soviet Union were uneasy allies; their collaboration was really the result of a mutual fear that the Nazis would gain control over Europe, not based on any ideological commonality. Consequently, after the war was over and the restructuring of Europe began, a power struggle developed between the Soviet Union (who wanted Germany to be Communist) and the United States and Britain (who wanted democratic rule.) However, you shouldn't make the assumption that devotion to ideology was all that was behind Cold War animosity; countries tend to be more complaint trading partners with countries that share their political systems and both Stalin and the Cold War Era presidents in the US knew this. The tension eventually built, but no one wanted to go to actual war again after the colossal massacre of WWII, hence the term Cold War.

Originally the Russians were only allies because of Germany being a common enemy. At the time there were 3 superpowers, Britain, America and the Soviet Union. Stalin was a terrible dictator and that killed an est. 20mil of his own ppl and of course they were communist so they were a natural enemy of the US and UK. During the end of WW2 Russia and the USA closed in on Germany. Once conquered it was divided between the allies. Both having very different regimes this of course created big problems (not enough space to right about that)

The main thing to fuel the Cold War was the Arms race. After the US successfully nuked Japan there was a race to master the technology and for who could create to most, not just nukes but everything defense related. Naturally everyone is building up arms with a finger on the launch button with huge tension. If one launched then bang...WW3

There was also the space race which was symbol of power and technological innovation.

There are so many contributing factors to the cold war, best to do a bit of reading up on it

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RE:

How did the cold war start after ww2 ?

During the latter half of WW2 Russia and the United States were allies, what happened to start the cold war

The United States and Britain never trusted the Soviet Union, even during the war. But they cooperated in order to fight the common enemy, the Nazis. Though after the war, the US and Britain gave sanctuary to a lot of former Nazis, because they had intelligence information on the Russians. The US began to mobilize their military, in order to meet what they perceived as a Russian threat to Europe.

On the Russian side, after losing 28 million people in a war that devastated their nations infrastructure and population, they were determined never to let that happen again. With the US and Britain giving sanctuary to former Nazi scientists and intelligence officers, and gearing up their militarily assets to possibly fight them in East Germany, the Russians decided to meet the threat.

The US spread a lot of propaganda about the Soviet Union, that they wanted to crush the "free world" and bring about a global Communist society. In reality, Russia was content to live within their borders using Eastern Europe as a buffer zone against the West, which had in the past killed millions of their people.

Why did the Soviet Union occupy Eastern Europe? A lot of those countries, like Hungary and Romania fought alongside the Nazis against them. Also if war was going to come again, Russia did not want it to take place on its soil. Better to turn Eastern Europe into the battleground rather than have the enemy pushing towards Moscow and laying siege to St. Petersburg, which happened in WWII.

Misunderstanding, misconceptions and propaganda all led to the Cold War. Also the fact that the US and Britain had no sympathy, compassion or respect for the sacrifice of the 28 million Russians who died saving the world from Nazism. That was the saddest chapter in the whole mess called the Cold War.

Russia trended toward Collectivism while the US recognized individual rights. The only way to manage the difference without destroying one or the other is by separation.