There were four powers that have won the WW2: the French, British, US and Russia - they had the power to occupy Germany and Austria after the war and decide about the conditions of the surrender of Germany and Japan.
With time, the four powers stopped working well together, and it became apparent that the first three had significant differences from Russia with respect to how the map of Europe should look like. Specifically, Russia wanted more control over the countries that bordered it. With time the differences grew bigger and Russia decided to exercise bigger control over the lands in the east, effectively establishing the "Iron curtain"