West Berlin was deep inside Soviet-controlled East Germany, giving the USA a foothold inside the Eastern bloc, and many East Germans crossed into West Berlin to reach West Germany
Between 1949 and 1961, 2.7 million East Germans crossed into West Germany via West Berlin, which looked bad for the Soviets as people clearly preferred West Germany
The Soviet Union knew that if it tried to push the West out of Berlin by force, a war would start that it could not win, so a series of summit meetings took place between the USA and Soviet Union
The Soviet Union knew that if it tried to push the West out of Berlin by force, a war would start that it could not win, as the US had more nuclear weapons
The USA refused to recognise Castro's government because it did not want a socialist country in their sphere of influence, especially not a country with close links to the Soviet Union
Became the Czechoslovakian leader in January 1968. He was a communist and supporter of the Warsaw Pact but wanted to make communism better and easier to live under. Dubček called this 'socialism with a human face'
Brezhnev and other communists in Eastern Europe, such as Eric Honecker, the leader of East Germany, were especially concerned. They feared the Prague Spring would lead to demands for reform elsewhere in the Eastern bloc that would threaten communist rule in Eastern Europe
Brezhnev now had a dilemma: Dubček was a friend and military action would damage the Soviet Union's reputation. On the other hand, if he did nothing, expectations would rise and the whole Eastern bloc might collapse