Newton’s third law of motion—to every action there is always an opposed and equal reaction—found profound resonance in the world of nuclear arms policy and negotiations. As Secretary of Defense Robert McNamara concisely articulated in a 1967 speech: “Whatever be their intentions, whatever be our intentions, actions – or even realistically potential actions – on either side relating to the build-up of nuclear forces, be they either offensive or defensive weapons, necessarily trigger reactions on the other side. It is precisely this action-reaction phenomenon that fuels an arms race.”