The Cold War had begun because each side had very different systems, communism and capitalism - Gorbachev’s policies of glasnost and perestroika were changing this.
On the night of November 9, East Germany’s Communist leaders gave permission suddenly for gates along the Wall to be opened. East Germans tentatively approached the border and found that border guards were letting people cross. Ecstatic crowds from both sides enjoyed a huge impromptu celebration. Some began chipping at the Wall with hammers and chisels. Official demolition didn't start until June 13th 1990 and in October of that year, the two Germanys merged to form a new united country.
Glasnost encouraged Eastern Europeans to speak out against communism and they overthrew their communist regimes. The USSR took no action.
In 1989, Gorbachev and the new American President George Bush were able to declare that the Cold War was over. In December 1991, the Soviet Union was dissolved and divided up into 15 separate independent countries. By international law, Russia was considered the successor state of the Soviet Union. This meant that it kept the nuclear weapons and the Soviet Union's seat on the Security Council of the United Nations.