Spies tend to make extravagant claims for their craft, but the reality of espionage is that it frequently makes little lasting difference. Politicians treasure classified information because it is secret, which does not necessarily render it more reliable than openly accessible information, and frequently makes it less so. If the enemy has spies in your camp, and you have spies in his, the world may be a little safer, but essentially you end up where you started, somewhere on the arcane and unquantifiable spectrum of “I know that you know that I know…”
Yet very occasionally spies have a profound impact on history. The breaking of the Enigma code shortened the Second World War by at least a year. Successful espionage and strategic deception underpinned the Allied invasion of Sicily and the D-day landings. The Soviet penetration of Western intelligence in the 1930s and 1940s gave Stalin a crucial advantage in his dealings with the West.
The pantheon of world-changing spies is small and select, and Oleg Gordievsky is in it: he opened up the inner workings of the KGB at a pivotal juncture in history, revealing not just what Soviet intelligence was doing (and not doing), but what the Kremlin was thinking and planning, and in so doing transformed the way the West thought about the Soviet Union. He risked his life to betray his country, and made the world a little safer. As a classified internal CIA review put it, the ABLE ARCHER scare was “the last paroxysm of the Cold War.”
Macintyre, Ben. The Spy and the Traitor: The Greatest Espionage Story of the Cold War. 2018. p. 182–3 (ellipses in original)
Related:
No doubt the atomic spies affected history too, Klaus Fuchs and the other Los Alamos and Manhattan Project insiders who aided and accelerated the Soviet bomb program.