We consider a game-theoretic model of counterproliferation, in which a single Incumbent (holding nuclear weapons) is faced with a sequence of potential Entrants (who are considering developing and deploying a nuclear capability or other weapons of mass destruction of their own). The Incumbent can either choose to act preemptively, with certain costs, or allow the Entrants to deploy a nuclear capability, with consequent costs due to the potential for nuclear blackmail. How should each of the players decide?The players' decisions are optimally based not only on their own cost functions (which each knows) but also on their opponent's cost structure (which is known only in probability). Entrants (other than the first) can learn something about the Incumbent's cost structure from decisions they have made in the past. It turns out that, depending on a would-be Entrant's cost structure, a record of preemption by the Incumbent can have either a deterrent or provocation effect. Understanding the basis for each of these contradictory effects is critical to designing an optimal counterproliferation strategy.