The 2011 Arab Spring is the story of what happens when autocrats prepare their militaries to thwart coups but unexpectedly face massive popular uprisings instead. When demonstrators took to the streets in 2011, some militaries remained loyal to the autocratic regimes, some defected, whilst others splintered. The widespread consequences of this military agency ranged from facilitating transition to democracy, to reconfiguring authoritarianism, or triggering civil war. This study aims to explain the military politics of 2011. Building on interviews with Arab officers, extensive fieldwork and archival research, as well as hundreds of memoirs published by Arab officers, Hicham Bou Nassif shows how divergent combinations of coup-proofing tactics accounted for different patterns of military behaviour in 2011, both in Egypt and Syria, and across Tunisia, and Libya.