What MOST led to such high tension during the cold war? (communism/containment, space race, cuban missile crisis, etc.) How could this issue have been resolved before becoming a bigger problem?

Communism

Because US and Soviet Union wanted world dominance or a capitalist world (US) or a communist world (Soviet Union).

The Cold War began shortly after World War II, as the United States and the Soviet Union contested political, economic and military hegemony in the world. A direct armed conflict would mean the end of both countries and, probably, of life on planet Earth.

This could have been avoided if no country wanted to rule the world.