Firstly, Nato did not exist before WWII - Nato was formed after WWII, partly to ensure it never happened again. However, the main attitude of the Western countries to Russia was distrust, though not the fear it became after WWII. Russia's declared aim was to spread Communism to the whole of the rest of the world - it viewed itself merely as the first country in an inevitable historic wave which would, in the fullness of time, take over the world as Marx had predicted. The West, of course, had many Russian refugees who had relatively recently fled the Revolution. They were only too happy to tell Western governments about the iniquities of the Communist regime. On the other, Russia was regarded as weak and distant, so it was not expected to be able to do serious harm any distance from its borders, though it might be a nuisance. The power built up by the USSR in WWII changed all this. Not only did it still have the declared intention of spreading world Communism, but it had delivered on that intention by taking over the eastern end of Europe. Its arms buildup during the war had given it a fantastic military production capacity - and then it developed the atomic bomb as well. Diffuse fear of a distant, rather weak country became paranoid fear of a large and powerful country with long range weapons and a huge army.