This is a fictionalised version of the early days of the CIA, from the point of view of Matt Damon's counter-intelligence expert. I'm not an expert on the history, but the impression I got was that the main characters were all loosely based on real people and likewise with the events in the film.
It's pretty slow, and Damon plays his guy so cold that it takes a while to find people in the film to empathize with. As everything starts to slot together, it becomes more compelling.
I'm not sure yet what message to take away from it, which at least suggests it's somewhat deeper than most films. Well worth a look.