Field Marshal Jan Christiaan Smuts (24 May 1870 - 11 September 1950) was a prominent South African and British Commonwealth statesman, military leader and philosopher. In addition to holding various cabinet posts, he served as prime minister of the Union of South Africa from 1919 until 1924 and from 1939 until 1948. Although Smuts had originally advocated racial segregation and opposed the enfranchisement of black Africans, his views changed and he backed the Fagan Commission's findings that complete segregation was impossible. Smuts subsequently lost the 1948 election to hard-line Afrikaners who created apartheid.

After nine years in opposition and academia, Smuts returned as deputy prime minister in a 'grand coalition' government under J. B. M. Hertzog. When Hertzog advocated neutrality towards Nazi Germany in 1939, the coalition split and Hertzog's motion to remain out of the war was defeated in Parliament by a vote of 80 to 67. Governor-General Sir Patrick Duncan refused Hertzog's request to dissolve parliament for a general election on the issue. Hertzog resigned and Duncan invited Smuts, Hertzog's coalition partner, to form a government and become prime minister for the second time in order to lead the country into World War II on the side of the Allies. Smuts had served with Winston Churchill in World War I, and had developed a personal and professional rapport. Smuts was invited to the Imperial War Cabinet in 1939 as the most senior South African in favour of war. On 24 May 1941 Smuts was appointed a field marshal of the British Army, 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 forward by Sir John Colville, Churchill's private secretary, to Queen Mary and then to George VI, both of whom warmed to the idea. In May 1945, he represented South Africa in San Francisco at the drafting of the United Nations Charter. Also in 1945, he was mentioned by Halvdan Koht among seven candidates that were qualified for the Nobel Prize in Peace. However, he did not explicitly nominate any of them. The person actually nominated was Cordell Hull.

Using a quote from the above article, answer the following question: How was Smuts involved in the second World War?
Smuts had served with Winston Churchill in World War I, and had developed a personal and professional rapport.