When the First World War broke out in 1914, the princess was visiting a friend, Anne Violet Cavendish-Bentinck (1864-1932), at the home of her mother, Mrs Louisa Scott; she immediately set out for France.

[1] She worked as a nurse,[2] at the family home of Chateau de Bellignies, which was in use as a hospital,[3] and assisted Edith Cavell in helping allied servicemen to escape to Britain via the Netherlands.

At their trial in October, she asked for clemency for her fellow prisoners, claiming that she and her brother were solely responsible.

[6] The Canadian journalist Elizabeth Montizambert, who had been in the occupied territory during the war, became a close friend of Marie and her brother.

Henri Giraud, who had been assisted by Nurse Cavell's team when he was a junior officer, once again received Princess Marie's help when he escaped from Königstein Castle in 1942.