This article develops a theory connecting security commitments and the decision to acquire nuclear weapons. In a threatening environment, third party security commitments can reduce a state's fear of abandonment in the event of war and its motive for acquiring nuclear weapons. However, a threatened state may reject at least some kinds of security commitments, such as foreign deployed nuclear weapons, if it fears that such commitments increase the risks of entrapment, the possibility that the threatened state will be dragged into a war it would like to avoid. The article looks at three kinds of security commitments, alliances, foreign deployed nuclear weapons, and foreign deployed troops. In quantitative tests, it finds strong evidence that foreign deployed nuclear weapons reduce proliferation motives, only very limited evidence that alliances reduce proliferation motives, and no evidence that foreign deployed troops reduce proliferation motives. It also presents several qualitative evidence, which supports the quantitative evidence, and in particular helps explain why alliance ties sometimes do not prevent proliferation.