The U.S. and Soviets cut nuclear weapons a few at a time so neither had to trust the other

If any rivalry looked like a pure win-lose fight, it was the Cold War between the United States and the Soviet Union. One side's gain was supposed to be the other's loss, in what game theorists call a zero-sum game.

Game theory is a mathematical way to make sense of these strategic encounters, where each side pursues its own goals. John von Neumann, who laid out the field with Oskar Morgenstern in their book Theory of Games, first built it to explain economic behavior like buying and working for wages. Later researchers extended it to biology, international relations, and even friendship and family life. One lesson from game theory research was that many interactions seen as zero-sum, or competitive, are "not as competitive as they seem."

Arms reduction shows how this worked. American and Soviet leaders realized both sides could cut the huge cost and labor of maintaining big arsenals if they each had fewer weapons. When Ronald Reagan and Mikhail Gorbachev negotiated the START Treaty, the obstacle was not the goal. It was verification. How could Washington be sure that while it was eliminating nuclear weapons, Moscow was doing the same, and vice versa.

Their answer was to stop treating disarmament as one giant trade. Rather than each side getting rid of a large share of its arsenal at once and hoping the other would follow, they divided the work into many tiny exchanges. The Soviet Union would eliminate just a few weapons, then the United States would eliminate just a few weapons. They would pause to verify, then continue, then eliminate a few more.

Because only a few weapons were at stake in any one step, each side could confirm the other's cut before making its own next cut. Neither side had to take the other's word for the whole arsenal at once.