Gorbachev agreed to remove his troops from East Germany to ultimately permit independence and reunification. In return, the US promised not to extend NATO any closer to the Russian border. The Russians fulfilled their end, but we did not fulfill ours. This is the cause of the current bad blood with Russia.
I always get an ironic chuckle when I hear people say Americans don't like war. Maybe the rank and file American doesn't like war but the financial/military/industrial complex sure does as they can't stay in business if there is no war.
