Didn't USA had the foresight that any country that embraces communism would eventually fail? Why did it go around spreading anti-communist sentiment to an extent where they defied the internal politics of countries and preferred to instill a dictator who would ruthlessly crackdown communist?
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US policymakers did not have total faith Communism would fail. They worried it would defeat capitalism, or at least coexist with it indefinitely.
For example, consider the "New Frontier" speech given by Senator (later President) John F Kennedy in 1960:
For the harsh facts of the matter are that we stand on this frontier at a turning-point in history. We must prove all over again whether this nation — or any nation so conceived — can long endure — whether our society — with its freedom of choice, its breadth of opportunity, its range of alternatives — can compete with the single-minded advance of the Communist system.
Can a nation organized and governed such as ours endure? That is the real question. Have we the nerve and the will? Can we carry through in an age where we will witness not only new breakthroughs in weapons of destruction — but also a race for mastery of the sky and the rain, the ocean and the tides, the far side of space and the inside of men's minds?
Furthermore, governments are concerned with the present as well as "the long term". The USSR lasted for 74 years (1917-1991). Cuba has been Communist for 58 years and counting (1959-). The US government wanted to have friendly regimes in other countries immediately, not many decades in the future.
Finally, the USSR collapsed in part because it was vigorously opposed by the USA. In particular, the Soviet government was unable to afford the costs of the arms race between the two nations. Of course there were other reasons for the USSR's collapse, but at the very least the arms race was an important contributing factor.
The Cold War was not just a struggle between two economical systems, but also a struggle between two hostile power-blocks: The US-lead Capitalist power-block and the USSR-lead Communist power-block. Both sides were considering the other side a direct threat to their existence. Even if one assumed that the system of the other side is unsustainable and will eventually collapse (which wasn't undisputed, see the answer by Royal Canadian Bandit for details): Until then the other block was still capable of starting another world war and likely willing to do so if they were sure they could win.
It was therefor important for both sides to prevent the other side from increasing their power by gaining more supporting satellite states. When a country allied with one side, that side gained exclusive access to that country's natural resources and economical power.
And even more important: Allying with a country gives the opportunity to station military assets on their territory. When you look at the geographical locations of important cold war hotspots like Vietnam, Korea or Cuba, you see their strategic importance.
This is why both power-blocks supported any regimes which favored their ideology and opposed those which favored the other's ideology.