Answer:
In my opinion it was Soviet Union, as they were trying to spread their ideology across the globe, sometimes even by interfering in the internal issues of certain countries.
Explanation:
Although, we can say that both countries contributed to the rise of tension, Soviet Union especially made things harder. They wanted to spread Communism, and didn't allowed civil liberties. On the other sides, they wanted to impose their strict regimes in the countries of Eastern Europe, and to destroy any type of rebellion against them.
By spreading their influence outside Eastern Europe they started endangering the states of capitalist democracy.
