The Truman Doctrine came out of Keenans Long Telegram where he prophases about the takeover with communism. Keenan being an American agent living in Russia, there is the question of authenticity. Can we believe everything written in that 8000 word telegram to be utter truth, or a simple example of a hyperbolic story? The man hated communism, he hated Stalin, this seems to be the perfect opportunity to bring both down, however, the telegram consisted of a basic post-war Soviet outlook, using evidence from the Soviets background history and so on.
It concluded that Russia had no aims to work with the U.S. and in fact, looked to destroy it (along with democracy). The contents of the Long Telegram seems to be the long term reactant that caused a major shift in American policy, perhaps giving the Truman Doctrine the authenticity and authority it needed to wage a more open and physical attack on communism. Without it, it is wholly unjustifiable, and would therefore not hold any weight in what it could do.
The Truman Doctrine itself is a declaration of U.S.A.s undying support for those free people resisting the threat or attack of communist takeover. It was also announced at a time when Britain could no longer fund the civil war in Greece, and geographically this was huge threat to Europe. Greece was not an island, and once communism had devoured the people there, what was there to stop it from spreading all over Europe?
To actually put the Truman Doctrine into play, Marshall Aid had to be introduced also. This was money that was available to any country in Europe that wanted to fight communism. Americas concern for Greece, and showing how pivotal it actually was to the fight for democracy, estimated the value of $350 million, compared to Turkeys measly $50 million. It shows that the fight for Greece was crucial, and it was clear, whoever had Greece had an advantage. This is probably why so much money was put into it.
The money was then expected to be used on economic acheievment, military and of course, ensuring political alliances. By doing this, America has realised its potential and ability it cannot destroy communism outright, so the only means of victory is through containment; bribing such countries would allow numbers and political power over them. The Soviet could try to oppose them, but ultimately, the force would be too great for them to succeed.
The reaction of the Truman Doctrine and Marshall Aid, led to the Soviet version of Cominform where Stalin convinced communist countries not to take any aid from the West, as in due time, they would be victorious against them. He then went on further to allow the Berlin Blockade 1948. Such actions however, do not make a huge impact on an international level. The West is obviously thinking about the long term, by introducing policy and aid, whereas the S.U. seems to be countering such attacks, doing the minimum needed to survive. How can this be a war (at this stage) if America is making numerous open attacks engaging as many allies as possible, and the Soviet is only defending its territory after the West has made a move?
This is also not the first time America has made the first move the Atlantic Charter of 41, the delay of the second front, the atomic bomb and so on. The most poignant is some historians views are that the Yalta and Potsdam Conferences in 45 were the beginnings of the Cold War. There was indeed a clash of personalities, the boasting of developments, and the disagreement of policy, but look at what they did achieve countries were allowed to elect their own governments, Stalin agreed to help in the war with Japan, Germany would be divided into four zones and all three were to join the U.N.
This shows that the big three wanted a solution with security and protection even though they were looking out for their own interest, they still managed to compromise an answer that would work for everyone. Consequently, with such a degree to agreement, this may not be a key reason for why the Cold War began.
If the Cold War did not begin before the Truman Doctrine, then events after it may have been a real starting point. The Berlin Blockade of 48 was an attempt to stop the West from accessing its part of Berlin, via land transportation (it also being a direct consequence of the Truman Doctrine and Marshall Aid). This went on for a year, and was a direct attempt to stop the spread of democracy into communist areas, however, why was it that for one whole year, that the West did not do anything about it? Yes, they were able to access Berlin via their airlift, but why not directly ambush Stalins efforts? A possible reason may have been that Eastern Europe was not a large threat, and could be dealt with as peacefully as possible and they were right, Stalin gave up after a year.
In conclusion, I feel that there have been many incidences that could have sparked off a war at any time, but the point is it did not. At this time, it wasnt a Cold War as such, but more tension, and build up of future events to come. Each side expected a level a muscle flexing, and was therefore prepared to take as much as they were willing to give. America was working towards a co-existence of communism and democracy, as well was Stalin (and this is evident in the Truman Doctrine). At any time, threat of termination was countered with a defensive act that would ultimately lead back to the equilibrium that suited them both.
However, I feel that it wasnt until the development of the atomic bomb by the Russians in 1949 that lead to the beginnings of the Cold War. America no longer had the upper hand, and fear of meeting their match may have been a short term cause for the declaration of the NSC-68, which took a more aggressive view on communism. It looked to roll-back communism, enforcing that they were successful. This in my opinion, is the beginning of the Cold War, and it was the Truman Doctrine (as well as other key events) that lead to this situation in the late 1940s.