Here’s a handy-dandy map that shows the location of Berlin in East Germany. The borders were heavily guarded and border guards on the East side had orders to shoot anyone who tries to leave. Lots of harrowing stories of people escaping this way or that. 

That’s how we ended up with the Iron Curtain; those were countries Stalin wanted the USSR to have control over.

That’s essentially it. And the results of that divide are still visible in Europe. Some anomalies to this were Austria and Finland. Austria somehow got insanely lucky and managed to negotiate a neutrality for itself, whereas Finland had put up enough of a fight to make it clear to Stalin that any attempt to take over the country by force would end up in a miserable, drawn-out quagmire of a resistance war.  Plus Stalin clearly perceived some advantage to having an officially neutral buffer zone there. That of course didn’t stop the Soviets from doing everything they could to bully Finland for decades to come. Never got real control over the country, though.
A different anomaly was Tito’s Yugoslavia. While both were communist and one arguably as bad as the other, Tito wasn’t willing to play Stalin’s lapdog, and had the means to back that up. There’s a well-known anecdote about the two, that after capturing yet another assassin sent by Stalin, Tito sent him the following message: 

“Stop sending people to kill me. We’ve already captured five of them, one of them with a bomb and another with a rifle… If you don’t stop sending killers, I’ll send one to Moscow, and I won’t have to send another.” 

Quite possibly apocryphal, but makes a good story. And also illustrates Stalin’s determination to rule over all of eastern Europe. At least. And a lot of the present day realities in Europe, especially the eastern side, can be traced back to that particular determination and its manifestations in policy.

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