Having spent their celebrated careers exploring this provocative question, John Ferejohn and Frances McCall Rosenbluth trace the surprising ways in which governments have mobilized armies since antiquity, discovering that our modern form of democracy not only evolved in a brutally competitive environment but also quickly disintegrated when the powerful elite no longer needed their citizenry to defend against existential threats.?

Bringing to vivid life the major battles that shaped our current political landscape, the authors begin with the fierce warrior states of Athens and the...