Since 1980 almost half of all armed conflicts have been fought between governments and ethnic minority groups over self-determination, yet surprisingly little is known about when and why these conflicts occur. The few studies that do exist focus on the deep injustices and structural conditions that may cause some groups to seek greater autonomy or independence and others not. I argue that ethnic groups are much more strategic than current theories allow. Ethnic groups decide whether to challenge based in part on whether the government has made concessions in the past, and whether the government can be expected to do so again in the future. Data on all ethnic groups for the years 1940 to 2000 reveal that ethnic groups are significantly more likely to seek self-determination if the government has acquiesced to an earlier group of separatists, and if the government is unlikely to encounter additional ethnic challengers in the future. Grievances and opportunity matter, but so does the larger strategic environment in which the government and its ethnic groups operate.I thank Jon Caverley, Rui de Figueiredo Jr., Tanisha Fazal, Zoltan Hajnal, Oliver Kaplan, Jack Snyder, and participants at the Program on International Politics, Economics and Security at the University of Chicago for their very helpful comments and suggestions; Ted Gurr and David Quinn for their detailed information about the CIDCM data set; and Kathleen Cunningham and Idean Salehyan for excellent research assistance. Finally, I wish to thank the National Science Foundation for their support in funding this research.