In 2002, U.S. President George W. Bush initiated the deployment of a new ballistic missile defense system. 1 The move triggered vociferous international concerns, including a recent statement of Russia and China condemning U.S. plans as a destabilizing move. 2 Indeed, the move amounts to a withdrawal from the 1972 Anti-Ballistic Missile treaty. The U.S. position is that such missile defense systems only reduce the damage caused by an incoming strike, and therefore do not threaten international stability. This paper provides a careful formal analysis of how the unilateral acquisition of defensive weapons may affect the sustainability of peace. We consider a dynamic game in which two symmetric countries repeatedly decide to be peaceful or to attack. Peace is sustained in equilibrium by trigger strategies in which attacks are followed by permanent conflict. Under complete information, peace is sustainable if and only if the value of continued peace is greater than the temptation of launching a surprise attack. Because defensive weapons limit the possibility of retaliation, the unilateral acquisi-