I study the relationship between the likelihood of a violent domestic conflict and the risk that such a conflict "externalizes" (i.e. spreads to another country by creating an international dispute). I consider a situation in which a domestic conflict between a government and a rebel group has the potential to externalize. I show that the risk of externalization increases the likelihood of a peaceful outcome, but only if the government is sufficiently powerful relative to the rebels, the risk of externalization is sufficiently high, and the foreign actor who can intervene in the domestic conflict is sufficiently uninterested in material costs and benefits. I show how this model helps to understand the recent and successful peace process between the Colombian government and the country's most powerful rebel group, the Revolutionary Armed Forces of Colombia (FARC).