After the start of the Boer War and the fall of Pretoria in June 1900, Smuts took to the field and led a Boer unit in the area of Vereeniging and Potchefstroom. At the Boer conference in June 1901, it was decided Smuts would lead an expedition into the Cape Colony. In early September Smuts and his unit crossed the Orange River near Zastron and entered Cape Colony. By early October they were in sight of Port Elizabeth and from here they turned west and by January 1902 Smuts and his men were in the western part of Cape Colony. The towns of Springbok and Concordia were taken and O’Okiep was soon under siege. Smuts participated in the meetings at Vereeniging and was there when the surrender was signed at the end of May 1902.
After the elections of February 1907, Smuts became part of Premier Louis Botha’s cabinet. In May 1910, the Union of South Africa was achieved under a constitution Smuts helped to write.
When WWI started, the British government thought it desirable if the South African forces could capture German South West Africa and destroy the powerful wireless transmitters there. Before joining the war on the British side, Premier Botha and Smuts had to put down an open rebellion in South Africa by units of their armed forces and some influential veterans of the Boer War. Premier Botha (once commander-in-chief of the Boer Army) returned to the field as General and led the South African and British forces supported by Major-General Smuts and forced the surrender of the German forces in July 1915. Soon, in February 1916, now Lieutenant-General Smuts, took command of the Allied forces in East Africa. Here he entered the chase for Colonel Paul Lettow-Vorbeck but Colonel Lettow-Vorbeck evaded capture even to the end of the war.
In January 1917 Lieutenant-General Smuts turned over command and left for England where he had been appointed a member of the Imperial War Cabinet. He resigned the War Cabinet in December 1918 and in late June 1919 he and Premier Botha signed the Treaty of Versailles.
Upon the death of Premier Botha in late August 1919, Smuts became Premier of South Africa and remained so until 1924. In September 1939 he was again Premier and was Commander-in-Chief of the South African and Rhodesian Forces rom 1940-1945. In September 1941 he was awarded the rank of Field Marshal. When the war ended in Europe in May 1945, Premier Smuts was in San Francisco working with other delegates on the United Nations Charter.
Field Marshal Jan Christian Smuts died at home in South Africa on 11 September 1950.