In what year did the Cold War start?

The orthodox view of the Cold War holds that it began in 1945 when the three great powers, the US, UK and USSR, divided up Central and Eastern Europe at the Yalta conference. However, there is dispute over whether what arose from the 1945 agreement can be classed as a war. While increased tensions rose from the division of Europe, they did not spill over into conflict until the 1949 Berlin Blockade, when the US had to resort to airdropping supplies to Berlin after Stalin had blockaded America from accessing their zone. Alternatively, some have argued that the 1951 Korean War was the start of the Cold War because it was the first time the powers engaged in armed conflict, albeit via proxy armies.