In the debate between offensive and defensive realism, a central issue is whether major powers can overcome the uncertainty that drives the security dilemma. Whereas offensive realists maintain that states cannot know others' motives and intentions, defensive realists argue that states can reveal their preferences by altering their military posture. Defensive realists have, how- ever, presented an incomplete account of the constraints and opportunities associated with military reassurance. To demonstrate its motives, a security- seeking state must take actions that will often increase its vulnerability to potential aggressors. Although offense-defense variables have been invoked to address the constraint of vulnerability, the conditions usually considered most favorable for reassurance—differentiation between offense and defense and an advantage for the latter—make it no easier to achieve. A defensive advantage makes reassurance difficult by encouraging all states to adopt defensive capa- bilities and by requiring large concessions to reveal benign motives. Only when offense and defense are differentiated and the balance between them is neutral can states reveal their motives without also endangering their security. These arguments are illustrated with three empirical examples: the Anglo- German naval race, Nikita Khrushchev's troop cuts, and Mikhail Gorbachev's arms limitation and arms control policies.