When politically marginal ethnic groups have transnational ties to co-ethnics in civil conflicts, the countries in which the marginal groups reside are more likely to intervene. Marginal-to-marginal group ties and marginal-to-dominant group ties between a group in a third-party country and actors in civil conflict increase the likelihood of military intervention. Familiar forms of ethnic ties such as politically dominant-to-marginal group ties and dominant-to-dominant group ties are merely special cases of ethnic ties between any group in a third-party country and co-ethnics in civil conflict. The article also presents evidence suggesting that ethnic ties affect how intervening countries take sides. Interveners tend to favour the government in civil conflicts when marginal groups in both countries have ethnic ties. Likewise, ties between two dominant groups are associated with government-biased interventions. In contrast, intervening countries whose dominant group has ties to a marginal group in civil conflict tend to favour the opposition. Overall, the results testify to the importance of including politically marginal groups’ ethnic ties when explaining how third-party countries behave.