 Following the outbreak of the Second Boer War, Cairns volunteers for service when his militia regiment was embodied in 1900.After peace was declared in May 1902, he left South Africa on board the SS "Bavarian" and arrived in the United Kingdom the following month.On 12 July 1894, Cairns married Olive Cobbold, daughter of the former MP for Ipswich, John Patteson Cobbold.