Anti-sanctuary efforts are sweeping the country, as the federal government and a growing number of states impose stringent restrictions on the ability of cities and other localities to limit their involvement in federal immigration enforcement. Many are now wondering how far this movement will go. But where and how did this movement begin? This Essay argues that the roots of the contemporary anti-sanctuary movement can be traced to Proposition 187, a ballot initiative adopted by California voters in 1994. As the nation’s first anti-sanctuary law, Proposition 187 established the basic provisions featured in nearly every anti-sanctuary measure enacted since. Moreover, it led the federal government to reshape federal law and initiatives to enable the kind of federal-local cooperation that Proposition 187 envisioned. As a result, Proposition 187 did more than simply set the groundwork for the modern anti-sanctuary movement. It also led to a restructuring of the federalism relationship that made the proliferation of anti-sanctuary legislation like Proposition 187 more necessary. In other words, although Proposition 187 is largely remembered as a benefit-restricting measure, it is its anti-sanctuary efforts that constitute its most lasting legacy.