Most major nonviolent civil resistance campaigns target autocratic regimes. Yet, most dictators are toppled by their close supporters, not civilian protesters. Building on theories of strategic interactions between leaders, security agents, and protesters, we make three core claims: first, protesters are relatively less likely to mount a major nonviolent uprising against dictatorships with personalized security forces; secondly, personalized security forces are more likely to repress realized protest; and, thirdly, security force personalization shapes the prospects for success of mass uprisings in promoting democratic transitions. We leverage new data on security force personalization—a proxy for loyal security agents—and major nonviolent protest campaigns to test these expectations. Our theory explains why many dictatorships rarely face mass protest mobilization and why uprisings that are met with violent force often fail in bringing about new democracies.