The Allied expedition to Salonika was a controversial campaign of the First World War that diverted French and British resources away from the Western Front. To sustain this expedition without depleting existing forces, the Colonial Office approached the High Commissioners of Australia, Canada, and New Zealand and requested that each dominion consider raising a Serbian military contingent for service in Salonika. In the decades preceding the outbreak of war, South Slavs had settled in each of the dominions and the War Office hoped to exploit nationalist aspirations for a pan-Slavic state and mobilise South Slavs in the dominions. In raising these contingents, dominion governments weighed between fulfilling a demand of the Imperial war effort and jeopardising domestic stability by empowering a culturally-distinct minority that was the object of public paranoia. This article will examine how the legal status of South Slavs changed in the three dominions as a result of these recruiting efforts along with the conditions under which South Slavs were able to volunteer for service in Salonika. A comparative approach reveals how Southern Slavs were defined and how they defined themselves as they navigated the categories of enemy aliens, friendly allies, and subjects of the British Empire.