The Capitalist American policy was containment. It was built on Kennan's long telegram which stated that the USSR would implode with time, if left to itself, due to it's corrupt nature. So the USA waited and did anything in their power to stop it from spreading into the Western European states. Kennan had also observed that it was more likely for countries to turn to radical left policies in situations of hunger and need. The fact that there was already proof in Europe to support this theory was one of the deciding points for Truman to set the Marshall Plan into place, Italy and France had both substantial Communist Parties and it was feared that in elections they might win fairly. Another factor was the fear of their own economical downfall. If Europe turned Communist, it would not take long for the USA to be left without trading options and backed against the wall. But containment of the Russian ideology was obviously something somehow offensive to Stalin as he thought the American ideology was the harmful one, and he thought that Truman was only acting in self-interest because of the benefits they were gaining from the situation.
An equally important cause in the development of the Cold War was the clash of ideologies between the two major powers, which made the Cold War inevitable when their common goal was lost. America and Russia emerged as super powers from the Second World War and their shared conviction that their beliefs and policies where the 'right' ones, that would prosper and help the world, were a reason they did not harmonize. Their beliefs were reduced to expansionism, and neither of the powers would give up their position in the world. Ideology was a very important cause in the Cold War and was such a big issue that it continued throughout it. It was furthered by the belief of having an 'enemy', binding the members of each group into one belief and making the antipathy greater. It was more important than the Truman Doctrine and the Marshall Plan because it was the main cause of the rivalry.
Another thing influenced by the conflict of ideologies was the territorial aggression between the two countries. The USSR and the USA both wanted to expand their empires, due to self interest as well as their ideologies. The USSR simply set up Communist governments in the countries they had liberated during the Second World War, Stalin's reasons where: they needed a 'buffer zone' to protect themselves from a vengeful Germany. America also attempted to expand but in a much more covert way, through the Marshall Plan, the Western European states became more dependant on the Americans, which gave Truman immense power overseas. Both Stalin and Truman saw the expansionism of the opposite as hostile action and their antagonism became more determined.
The territorial expansion and the ideological clash caused an arms race between the two, re-armament of the infantry, development of their air force and finally, invention of nuclear weapons where a part of this. The arms race made an actual war seem possible and in the future, even though the possession of nuclear weapons on both sides made this impossible really. There were also misconceptions about the other's armies that furthered the Cold War, Stalin and Truman where so caught up in it they thought their enemy was more powerful than they actually were and tried to rise to the non-existent occasion. The arms race escalated within a year to develop the atomic bomb, before the Truman Doctrine and the Marshall Plan where even thought of. But that was not enough they kept searching for weapons that could give them an advantage and a few years later they developed the hydrogen bomb which assured mutual destruction if there was to be a war. The arms race made the idea of an enemy and a heating up of the situation more imminent, which alienated the other state even more.
The Truman Doctrine and the Marshall Plan played a great role in the further development of the Cold War, but it was not the most important. At least as important was the clashing of ideologies and what it brought with it. The arms race and territorial conflicts were also part of the development. All in all, the Truman Doctrine and the Marshall Plan where a force propelling the two states apart and setting them on opposite sides of the scale. It was a very important factor in the carrying on of the conflict as it furthered the inability to compromise of the two states. It's importance in the development of the Cold War was only rivaled by the ideological clash between the two states. It lead them from simple mistrust to hatered and loathing.