Democracies often go to war but almost never against each other. Indeed, "the democratic peace" has become a catchphrase among scholars and even U.S. presidents. But why do democracies avoid fighting each other? Reliable Partners offers the first systematic and definitive explanation. Examining, decades of research and speculation on the subject and testing this against the history of relations between democracies over the last two centuries, Charles Lipson concludes that constitutional democracies have a "contracting advantage" - a unique ability to settle conflicts with each other by durable agreements.