Cold War.

Political. I think that everything stems from political causes.  The United States was against communism and were frightened about what would happen if communism hit the US. I think that this "red scare" led to the US wanting to eliminate communism world wide so that they would not be the minority or the only non communist nation.  While the red scare could be considered a social problem it stemmed from a political aspect and therefore i would classify it as a political problem.  Many developing countries in Asia like Korea and Vietnam did not have a stable political system in place at this time, or internally had conflicting systems of government which lead to US and USSR intervention.  That lead to tensions between the two countries leading to the Cold War.
I don't think that we can pinpoint a cause of the Cold war. I think that the cold war was a gradual thing not one thing created the cold war.  it was more of a general opposition.  The US and the USSR were constantly in conflicts and these tensions lead to greater feelings of hate and when technologies grew it lead to a stand off.  It wasn't over one specific even but many events combined and the political tension between the two nations. 
They are both to blame.  There are two sides to every war, you cant just blame one country.  I think that the conflicting political views got these two counties involved in the same developing countries which lead to the tensions.  Both countries got involved in other counties and tried to spread their views and therefore it is not one or the other its both.