Recent literature attributes the relative scarcity of open internation
al markets to the prisoner's dilemma structure of state preferences wi
th respect to trade. We argue that the prisoner's dilemma representati
on does not reflect the most critical aspect of free trade agreements
in an anarchic international system, namely, their security externalit
ies. We consider these external effects explicitly. Doing so leads us
to two conclusions: (1) free trade is more likely within, rather than
across, political-military alliances; and (2) alliances are more likel
y to evolve into free-trade coalitions if they are embedded in bipolar
systems than in multipolar systems. Using data drawn from an 80-year
period beginning in 1905, we test these hypotheses. The results of the
analysis make it clear that alliances do have a direct, statistically
significant, and large impact on bilateral trade flows and that this
relationship is stronger in bipolar, rather than in multipolar, system
s.