Which of the following has the best claim to mark the start of the Cold War: Churchill's Iron Curtain Speech; Truman Doctrine; Berlin Blockade?
There is an argument to be made for each of these events and there is no way to objectively say which one should be seen as the beginning of the Cold War. What I will do here is to lay out why each of them could be seen in that way. You can then decide for yourself which one is really the start of the Cold War.
Churchill’s speech can be seen as the start of the Cold War because it was the beginning of the truly antagonistic mindset on the part of the West towards the Soviet Union. Since the Soviets had been an ally in WWII, there were many who did not see them as the enemy. Churchill’s speech represented a change in attitude towards the suspicion that permeated the Cold War.
The Truman Doctrine was the first overt action of the Cold War by a superpower. Truman announced that the US would act to prevent countries from falling to communism. This committed the US to a policy of containment. This was the first time that one of the superpowers came right out and said they meant to take action to contain the other side.
The Berlin Blockade was the first time that a superpower took overt action against the other superpower. The civil wars in Turkey and Greece were not overt actions by the Soviet Union. The blockade was. This was the first time that a superpower was willing to act in such a way.