On September 6, 1939, the Union of South Africa declared war on Germany, officially joining the Allies in the struggle of World War II. This announcement came just days after the United Kingdom had declared war on Germany in response to the invasion of Poland. The decision to enter the war was largely influenced by Prime Minister Jan Smuts, who argued for South Africa's responsibilities as part of the British Commonwealth and the need to oppose the ideological threat posed by Nazi Germany. His call to arms resonated with those who supported the Allies but faced opposition from factions within South Africa that were either isolationist or opposed to British control.

In the days leading up to the declaration, public debate intensified around the implications of war. Many South Africans were aware of the sacrifices that awaited in another global conflict, especially in light of the recent memory of World War I, where South African troops had fought alongside British forces. The nationalist sentiment in the country also added complexity to the decision, with various groups advocating for differing positions regarding allegiance and participation.

The Union's involvement in the war significantly impacted its military and political landscape. South African troops fought in various campaigns in North Africa and Italian East Africa, contributing to the international war effort. Additionally, the war catalyzed changes in domestic policies, including issues related to race, labor, and national identity as the country grappled with its role in a global conflict while managing internal divisions.