The Left-Wing Peace & The Right-Wing Peace

How strong is the nexus between domestic and international politics? The democratic peace contends that democracies do not fight interstate wars against each other. We argue there is also a “left-wing peace” and a “right-wing peace,” where people are less likely to support the use of force against countries that share their left-wing or right-wing ideology. This effect holds independent of whether a foreign country is democratic or nondemocratic. Observers view out-ideology states as more threatening than in-ideology states due to out-group bias, which is exacerbated by increasing polarization. Two survey experiments among the American public, and an analysis of militarized interstate disputes, provide strong evidence for our argument. Our theory connects major literatures across political science subfields, including social identity theory, political polarization, and the democratic peace. It significantly contributes to debates about whether politics “stops at the water’s edge” and the impact of polarization on political violence.