In order to follow the change in American policy over the defined period of time, during Second World War (WW II) and after war period, it is necessary to understand American general foreign policy.
I must say that obviously that change in policy led to Cold War, but according to historical resources, American government did not wanted it, but it was inevitable, as its absence could lead to overall insecurity and instability in the world. WW II played the key role in reorientation of the foreign policy of United States. Before the conflict of interests that took place, US leaders believed that the best way to protect their country from attacks and sustain peace was to minimize political and overall influence and attention overseas.
But events of the beginning of 1940s changed their views on the contrary. The key moment for making the ultimate decision about changing of the external policy was Pearl Harbor. From that event, US government decided to reconsider its view upon security items and try to struggle for it through involvement and not isolation. US leaders had come to such an idea, that if they do not interfere into world events, new wars can start, and therefore they have decided that the general system of the relations between all nations should be analyzed, structures and reconsidered, they were looking forward reforms. And it was obvious that it was America to which shoulders this task should be put.
This shift of foreign policy may have had some more deep and hidden reasons. It was WW II; Soviet Union was looking forward determination of spheres of influence. Probably US just didn’t want to stay away when the world was dividing into part upon spheres of control, but it is my subjective opinion.
USA leaders have decided to take into consideration prior experience of how things were going one in the world, and that greatly influenced their vision of the future and their future expectations. US government primarily wanted to avoid mistakes, which upon their opinion led to the WW II, therefore its strategy was to disarm defeated enemies and provide to all people an opportunity to choose and shape their own future taking into consideration their own experiences and beliefs. US leaders policy was changed to striving for the revival of the world trade and wanted to create a unique and effective organization, which would be involved with the security issues, instead League of Nations. But this plan was absolutely meaningless when the Axis was still not defeated. And taking into consideration the military situation of that time, it became obvious that this plan could only be implemented through the cooperation with USSR.
But the ideas and views of the Soviet Union differed from that of the US government. And the change of American foreign policy was not very much welcomed. Stalin, as a USSR leader, also reviewed experiences of the past, but the conclusions were different from those that US leaders made. Stalin was not interested at all in corporate security items, and didn’t see any sense in the reformation of the monetary system. But Stalin could not accept self-determination of the Eastern Europe, as this region was very much important to the Soviet Union and to its overall security.
So, two key premises- Eastern Europe and Germany and its allies, became major points of discord for the beginning of the Cold War.
The greatest anxiety of US leaders was stipulated by Soviet Union strong desire to spread its ideology of capitalism throughout the world, as well as to establish communism, though those attempts were not always successful.
So, it is possible to see how from being neutral to USSR, with the change of the direction of the foreign policy, USA and USSR appeared to have conflicting interests and views upon post-war period and overall economic development. USA had a strong advantage, which it wanted to use on their path in having the strategic view of the world reforms with USSR- the atomic bomb.
US officials were absolutely sure that this type of weapon would make USSR accept US peace plans and lay the ground for future fruitful cooperation. But Soviet Union refused, and this kind of strategy through force pressure had failed.
USA obviously wanted not only to establish cooperative relationships with USSR, but also wanted to spread its economic influence upon it. Therefore, that open policy of the United States was just the protection of its rights and step-by-step domination establishment on the world arena. US foreign policy was stipulated not only by the economic interest, but also was defined by the internal political system, which was the basis of economic determinism.
It is possible to argue very long upon who was right and who was wrong, as well as about reasons of the Cold War and who is guilty in starting it. But none of the arguments can be evaluated objectively as USSR and USA had different backgrounds, stages of development, history and spheres of interest. To put the responsibility on one of the sides, it would be necessary to estimate all opportunities of them to prevent this War, what was done and want was not.
In the conclusion, I would like to summarize that the reason for the Cold War was the interaction of different circumstances, which could not be controlled by those both powers, as well internal influences of the situations in USA and USSR.
And it cannot be evaluated objectively now whether the change of US policy had positive or negative consequences, but the obvious is one thing- the policy was drastically reconsidered by US leaders and it was changed from the policy of isolation to the policy of interference and taking active part in all actions USSR and Axis were involved in.