The Nuclear Arms Race is the main event of the cold war which happened in the 1950´s . The US beat USSR in the invention of Super bomb which was Hydrogen related or based off of. The Thermonuclear bomb was much stronger then the Atom bomb. That helped the US until the USSR created one of their own to get back at US. During the Cuban missile crisis the tensions with the US and USSR never been any higher an all time high. The thermonuclear bomb was said to be a 100 times more than the atom bomb.
This chart shows the amount of nuclear warheads in the U.S. and the U.S.S.R. from 1948 to 2012. The US was the only country to have nuclear weapons during WW2 therefore had bragging rights in terms of power over their main rivals ( Japan and later the USSR). In 1949 the USSR had their own version of the atomic bomb, the the arms race and the cold had began.
As shown here, these were the most powerful bombs at the time. The Tzar bomb was equal to 50 tons of TNT. This massive bomb was created by the Soviet Union and was named the "King of bombs" for pretty obvious reasons.
RESOURCES
http://www.history.com/topics/cold-war/arms-race
https://onestopcoldwarshop.wikispaces.com/Nuclear+Arms+Race