Part of the point of le Carré is that the stakes aren't clear, and whatever they are, they're often not worth the shoddy business the Circus constantly involves itself in. The Cold War is a looming cataclysm, of course, but these fucking spies are just running around immiserating people in the (probably ill-founded) hopes that they're making the world in any way better, when really all they're doing is picking which organized crime family gets to run the heroin business in Laos.

The work Smiley does might be incredibly important, or it might not be. He himself grapples with that ambiguity (and with his oscillating affiliation with the Circus), which is part of what drives the narrative.

So the answer to your question here is basically like asking "who's the murderer" in an Agatha Christie book.