Germany was defeated by nations around the world and now U.S.A, Britain, and Russia have no reason to be friends anymore. They are now doing their own things and have each taken parts of Germany for themselves. Russia always had a rocky relationship with U.S.A and Britain because of there differences.
Russia is helping countries that were damaged by the war get back up by slowly making them communist countries and giving them resources they need. U.S.A doesn't like the countries slowly beginning to become communists and wants to do something about it.
America starts to get angry about the countries becoming communists and they create a organisation called the North Atlantic Treaty Organization or NATO. It was made to help western countries like Canada, USA, and 10 more countries in Europe. It protected countries that were attacked during the war by giving supplies and to stop Russia from making more countries communists. America is no longer friends with Russia
Russia signed a treaty and then created a group called the Warsaw pact which was made to oppose against NATO and to make communism a bigger thing around the world. The members of the group were all soviets
Russia has tested their first nuclear bomb in 1949 and it was named first lightning and America has taken this as Russia threatening them and are getting ready to use their bombs on Russia before testing it.
Russia and America are both insisting they will use their Nuclear weapons on each other and will destroy each other and are bot getting very angry and are almost ready to use their weapons on each other although if they did the effect of the bombs would still last till today.