1) United States of America.
2) Nearly 30,000.
3) i) Ideological conflicts between US and USSR.
ii) For military supremacy and to gain economic supremacy lead the countries to emerge camps after the Second World War.
4) i) Realised that war mongering and the consequent arms race only made the world more unsafe and increased the possibility of a disastrous war for all countries.
ii) As a result of the pressures the USA and USSR, the main competitors in the arms race, signed on cut down their nuclear arsenal (SALT, START).
iii) Cold war came to an end with the collapses of USSR in 1991; is also another reason for this.