Many supporters of open borders argue that restrictions on immigration are unjust in part because they undermine equal opportunity. Borders prevent the globally least-advantaged from pursuing desirable opportunities abroad, cementing arbitrary facts about birth and citizenship. In this paper I advance an argument from equal opportunity to global freedom of movement. In addition to preventing people from pursuing desirable opportunities, borders also create a prone, segregated population that can be dominated and exploited. Restrictions on mobility do not just trap people (...) in bad opportunity sets—they help create bad opportunities by isolating the negative externalities of production and foreign policy. Freedom of movement can play a vital role in spreading risks and burdens, incentivizing their mitigation. Using an analysis of feudalism, segregation, and the transnational economy, I illustrate the centrality of space and mobility, showing why freedom of movement is a necessary tool for preventing political and economic oppression. (shrink)