Mercenary armies went out of style in the nineteenth century; it became com
mon sense that armies should be staffed with citizens. I argue that even th
ough realist explanations focusing on the fighting prowess of citizen armie
s and sociological explanations focusing on the fit between citizen armies
and prevailing ideas can rationalize this change, they cannot explain it. I
examine, instead, the politics behind the new reliance on citizen armies a
nd argue that material and ideational turmoil provided important antecedent
conditions for change. Beyond this, individual states were more likely to
move toward citizen armies when they had been defeated militarily and when
the ruling coalition was split or indifferent about the reforms tied to cit
izen armies. Finally, the apparent success of citizen armies in France and
then Prussia made domestic conditions for reform easier to obtain in other
countries, reinforcing the likelihood that the solution would be replicated
. I conclude that the interaction between domestic politics and path depend
ency provides a promising source of hypotheses for explaining the condition
s under which new ways of war emerge and spread.