Conventional analysis of military mobilisation usually focuses on the organisation and structure of armed forces composed of the citizens of nation-states. With the rise of contemporary insurgencies and the proliferation of Islamist movements such as the Islamic State in Iraq and Syria (ISIS) or Al Qaeda in Islamic Maghreb (AQIM), which includes military service by foreign nationals, there has been a growing focus on the phenomenon of transnational fighters. However, the phenomenon of transnational volunteers for military service in foreign conflicts has a long history. This was particularly evident in large-scale conflicts such as the First and Second World Wars. This chapter explores the phenomenon of volunteering for foreign wars by focusing on the dynamics of the enlistment of foreign nationals in the Union of South Africa during the Second World War (1940-1945). It seeks to establish why and how South African authorities enlisted foreign nationals in the country's Union Defence Force (UDF), and why foreign nationals decided to volunteer for military service in the Union's armed forces. The chapter broadly analyses the Union's defence policy, military mobilisation challenges, recruitment and rationale for enlistment of foreign nationals. This analysis is framed within the context of South Africa's internal political divisions at the time and anti-war resistance which impacted on the country's war effort during the Second World War.