To deserve serious consideration, a strategic defense system must pass four tests: (1) it must be technically feasible. (2) It must preserve the war avoidance stability of mutual deterrence. (3) It cannot be so expensive that an adversary can cheaply overwhelm it. (4) It must be politically feasible. Historically, proposed strategic defenses have failed all four tests. But recent changes could make strategic defense prospectively viable if provided as a global public good.