Why did containment become the basis of policy for the USA from 1947?
Containment became US policy in the form of the Truman Doctrine, partially created to help countries that rejected communism but didn't have the means to fight the USSR, and also to help protect the USA's security fears.
Stalin had began to establish a new national security system based on the infiltration of Russia's surrounding states, turning them into satellite states that were 'independent' however under the control of the USSR. The Soviet's aimed to enhance this system further by increasing the amount of satellite states they had, thus expanding beyond Europe, deciding on Turkey first as this would give them a huge amount of control over the Mediterranean. The USA wouldn't let this happen and so began to give Marshall Aid to European countries in the hope that they could build up their economies and help them recover from the war; they reasoned that if people were prosperous they would be less likely to turn to the communist ideology.
The policy of containment had to influence the American government to break away from isolationism and stop the threat of communism. By giving away so much foreign aid to prevent communism, it raised tensions between the US and USSR as both ideologies heavily opposed one another. Russia also believed that the USA were expansionist and by doing this the US could develop their global economic power by making people both militarily and economically independent on them and the trade they provided.
The policy of containment meant that the US would, via military means, stop the spread of communism, but wouldn't interfere with countries who already had communism or who wanted it. However, they would interfere with any countries that opposed communism and were having it...