Captured as a prisoner of war by Imperial Germany in 1914, he was held in captivity for two years before appealing to the Kaiser for a visit to his dying mother.

His request was granted and after a two-week visit he voluntarily returned to the POW camp, where he remained until the end of the war.

The British Expeditionary Force (BEF) was engaging the Imperial German Army during the Battle of Mons.

[1] He spent two years in captivity, at a POW camp in Magdeburg, north-east Germany, when he got word from his sister that his mother, Louisa, was dying.

Richard van Emden, writing in his 2013 book Meeting the Enemy, speculated that Campbell would have felt it honourable to return and "he would have thought 'if I don't go back no other officer will ever be released on this basis'".