US, Britain, and France were allies during WW2. The USSR joined following the execution of Operation Barbarossa by Germany. Even though it was promised that this alliance would continue after the war, tensions began to rise. The US and the USSR became the first and second most powerful countries in the world, respectively.$^2$

USSR looked favorably upon communist countries, and many that weren’t were replaced by communist governments by the USSR. However the US and other western countries saw communism and by extent, the USSR as a threat their way of life and freedoms.$^2$

Hence, the world split into two diametrically opposed blocs. The communist powers, led by the USSR. The powers that opposed communism, led by the US. Eastern and western Europe respectively.$^2$

Both, US and USSR, began to look at any development in the World as either for or against communism. Both countries went to extreme extents to prevent the development of the opposite sides.$^2$

Another cause for tension was the arms race that they both underwent. They attempted to outdo the other while preventing the other from knowing what they have developed.$^2$