This is "The Defenders" by Philip K. Dick, first published in Galaxy Science Fiction, January 1953.
To survive a nuclear war, humanity has retreated to vast underground bunkers, each of which is a city or at least a moderate-sized town in size. (The protagonist is reading the daily newspaper in the first scene.). Life goes on in the bunkers as the people work to produce more bombs, planes and especially combat robots known as "leadys" built to survive the atomic wasteland above.
But today someone has gotten suspicious; summoning a command leady from the surface for a briefing, they disable it and find no trace of radiation on it at all. An expedition to the surface is planned.
Bursting onto the surface, they discover a pleasant, green, peaceful world, with no trace of the radiation storms they've been shown for all these years. Instead of fighting, the robots have been cleaning up and restoring the world.
Confronted, the robots admit the deception and claim their calculations say that people on both sides are tiring of the war and will soon be willing to give up and come up and live in peace.
The leaders try to get down into the bunkers, but the leadys have already sealed the entrance; they will claim a direct strike destroyed it so the masquerade can continue for now.
The expedition gives in, and goes to live on a restored farm, where they meet a team of their Soviet counterparts who had the same experience.