In the South African War, Smuts commanded (1901-2) Boer guerrilla forces in the Cape Colony. By 1904 he concluded that the cooperation of Boer and British elements was essential to the greatness of South Africa, and he joined with Louis Botha to achieve this alliance. Smuts was instrumental in the creation (1910) of the Union of South Africa (see South Africa). Smuts continuously held office in Botha's cabinet, serving as minister of defense (1910-19), of interior and mines (1910-12), and of finance (1912-13). His use of military force and of admittedly illegal deportations in breaking a miners' strike cost him the support of labor.

Early in World War I, Smuts smashed a new Boer uprising, and in 1916 he served successfully as a general in South Africa's campaign against German East Africa. He was a member (1917-18) of the imperial war cabinet in London, and he signed the Treaty of Versailles. However, he protested that its terms would outrage Germany and prevent the harmonious world order that he believed could best be served by the League of Nations.

Upon Botha's death (1919), Smuts headed the United South African (Unionist) party, and from 1919 to 1924 he was prime minister and minister for native affairs. Weakened by his frequent absences and another strike-breaking incident, his party lost the election of 1924 to a coalition of labor and anti-British nationalists. Smuts in retirement wrote Holism and Evolution (1926, 3d ed. 1936), in which he developed the view that evolution is a sequence of ever more comprehensive integrations; in the political sphere the British Empire and the developing world community provided the highest examples.

Smuts was (1933-39) minister of justice in a coalition cabinet, but when Prime Minister Hertzog opposed entering World War II, Smuts became prime minister. In 1941 he was created field marshal. He spent most of the war in London, where he had a high place in the British war councils, and he was very active in organizing the United Nations. In South Africa, however, Smuts's party lost the election of 1948 to the Nationalists. Smuts represented that portion of South African sentiment that stood for cooperation with the British Empire and that had somewhat less extreme racial views than the Nationalists.