Which event do you believe started the Cold War? See description please.?

I do not think an event started the Cold War; it was a stand-off which developed in response to irreconcilable interests rather than a specific conflict. Marxism and liberalism (and yes America is a liberal country despite the tendency there to make that into a dirty word.) The West felt threatened by socialist ideas which had considerable support in intellectual circles even after the crushing of the revolution in Hungary. Stalin was notoriously unwilling and unable to trust anybody and had that mistrust confirmed by invasion from Germany. He had acquired most of Central and Eastern Europe and needed to cut it off from western europe to maintain the puppet Govts installed there.