What drives governments to crack down on and kill their own civilians? And how--and to what extent--has nonviolent resistance historically mitigated the likelihood of mass killings? This special report explores the factors associated with mass killings: when governments intentionally kill 1,000 or more civilian noncombatants. We find that these events are surprisingly common, occurring in just under half of all maximalist popular uprisings against states, yet they are strongly associated with certain types of resistance. Nonviolent uprisings that do not receive foreign material aid and that manage to gain military defections tend to be the safest. These findings shed light on how both dissidents and their foreign allies can work together to reduce the likelihood of violent confrontations.