In March 1915 Marthe met Christopher Thomson, the British military attaché, at a Palace soirée; he was arranging for Romania to join the Allies (although he did not agree with the policy, as Romania was unprepared for war). He remained devoted to her for the rest of his life. They corresponded regularly, and she dedicated four books to "C.B.T." Later he was a Labour peer, and Secretary of State for Air. She visited the site of his death in the R101 airship accident on December 1930 with their mutual friend the Abbé Mugnier.