Heya,
you might want to first define what the "Cold War" is and it would essentially boil down to about 4 main issues:
1) Difference in ideology
2) Superpower rivalry
3) Formation of rival blocs
4) (which would become more apparent later on) indirect conflict
It is true that fear does drive the parties to action but the fear itself can be subsumed under the "difference in ideology". Same goes for the breakdown of the Grand Alliance which is in turn driven by mistrust and fear. Another thing to note about wartime tensions is that by itself it laid the foundations but DID NOT start the Cold War. You must bring in the subsequent post-war events in order to show how the development of the tensions would eventually bring about the realisation of the Cold War between USA and USSR.
The appearance of these issues could be seen though the Yalta & Potsdam conferences, Sovietization of Eastern Europe, Truman Doctrine, Marshall Plan, Baruch Plan, 1st Berlin Crisis, formation of NATO and later the Warsaw Pact. Your job then is to argue which of these events would prove to be the most critical in building Cold War tensions (as defined the 4 issues)
However, if the essay question does not ask which event was the most critical in contributing towards the outbreak of the Cold War, you will then need to consider the 4 School of Thoughts - Traditionalists/Orthodox, Revisionists, Post-Revisionists and Post '91 Revisionists. This approach will then require you to analyse the complex relations between USA & USSR especially their actions and counter-reactions (i.e. formulation of Marshall Plan was in response to the growing Soviet influence in Eastern Europe)
Hope this helps