What year did the Cold War start?

While friction between the United States and the Soviet Union began shortly after the end of World War II, the Cold War itself is generally understood to have begun in 1947. This was the year that the United States made several moves that caused heightened tension with the Soviets. First, it began the Marshall  Plan, a program that gave massive amounts of aid to Western European countries to help them rebuild after the devastation of the Second World War. This brought these countries firmly into the United States' sphere of influence. It was also the year that President Truman unveiled the Truman Doctrine, calling openly on Congress for aid to help fight the spread of communism. Truman specifically wanted funds to help keep communism out of Greece and Turkey. The Soviet Union obviously took the Truman Doctrine as a hostile move and reacted against it by solidifying its own control over Eastern European. It installed communist regimes throughout the region, which also increased tensions. 
https://www.britannica.com/event/Cold-War