https://en.wikipedia.org/wiki/Jan_Smuts
"He led commandos in the Second Boer War for the Transvaal. During the First World War, he led the armies of South Africa against Germany, capturing German South-West Africa and commanding the British Army in East Africa. From 1917 to 1919, he was also one of five members of the British War Cabinet. He became a field marshal in the British Army in 1941, and served in the Imperial War Cabinet under Winston Churchill. He was the only person to sign the peace treaties ending both the First and Second World Wars."
"Smuts' importance to the Imperial war effort was emphasised by a quite audacious plan, proposed as early as 1940, to appoint Smuts as Prime Minister of the United Kingdom, should Churchill die or otherwise become incapacitated during the war. This idea was put by Sir John Colville, Churchill's private secretary, to Queen Mary and then to George VI, both of whom warmed to the idea."