The Truman Doctrine was a proclamation issued by Harry S. Truman on the 12th of March 1947. It ensured that the USA would provide economic, military as well as political support to all countries that had been undermined by oppressive regimes (in the context of the time period what this meant was Communism). This was a major break away from conventional American foreign policy as previously the country had adopted a policy of isolationism in order to bolster their still very new economy and stray from the affairs of the rest of the world. This inspired the change from détente (a relaxation of tension) to a policy of containment of Soviet expansion.
In terms of what Truman and his advisors wanted, this doctrine was perfect as it now gave the USA an excuse to intervene in what they deemed as tyrannical Soviet regimes. The US public however were not entirely fond of the idea of getting involved in Europe again as they deemed the USA’s involvement in WW2 to have caused harm to their own economic wellbeing.
The cause for the sudden change in policy and the abruptness of the speech came from a recent announcement from the British government that as of the 31st of March they would no longer be funneling money and military support into the Greek Civil War against the Greek Communist Party. Britain was made bankrupt by the financial strains of WW2 but also the winter of 1946-7 which caused severe hardships in terms of the economy, and living conditions. Because of Britain’s inability to fund their campaigns in Greece and Turkey, in February 1947, Britain formally requested for the United States to take over its role in supporting the Greeks and their government. The policy won the support of Republicans who controlled Congress and involved sending $400 million in American money but no military forces to the region.