The post-cold war era has seen an unmistakable trend toward the proliferation of violent non-state groups-variously labeled terrorists, rebels, paramilitaries, gangs, and criminals-near borders in unstable regions especially. In Borderland Battles, Annette Idler examines the micro-dynamics among violent non-state groups and finds striking patterns: borderland spaces consistently intensify the security impacts of how these groups compete for territorial control, cooperate in illicit cross-border activities, and replace the state in exerting governance functions.