Essentially, Containment was America’s policy designed to win the Cold War without recourse to war.
It was the brainchild of an American diplomat stationed in Moscow: George F. Kennan. He argued that, to defeat communism and the Soviet Union, yet avoid war, a long-term policy, requiring patience though firmness, was required. What they had by 1947, notably Eastern Europe, they had. America couldn’t do anything about it unless it was prepared to go to war, which it wasn’t. But America could draw a line in the sand and contain communism, starve it of influence and more importantly, of trading partners. This would in turn, starve its people of the material benefits enjoyed in the West, and this would in turn bring about the collapse of the Communist regimes from within, i.e. by the hands of their own peoples.
Containment began in Western Europe with the Truman Doctrine and Marshall Aid. America was aware that in post-war Europe, communism might seem attractive as people suffered from the consequence of war, and as Communists had often played a leading role in the resistance movements that defied Nazi rule.
But it was also appealing in states that had been subjected, or were still being subjected, to imperialist rule. Again, these states were often poor and certainly hadn’t been treated as equals by the imperial powers, and communism held a strong appeal. So, America encouraged Britain and France to withdraw from empire and supported the newly independent countries with aid, in so doing making these newly independent countries reliant on that aid as well as American-led military treaties.
It was indeed a long-term policy but America was confident that its economy would stand the test, whilst that of the Soviet Union’s would eventually collapse which of course, helped by the astronomical costs of military spending in the nuclear arms race, it did.