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).
When Romania at last entered the war on the Allied side in 1916, Marthe worked at a hospital in Bucharest until the German army burned down her home in Posada, in the Transylvanian Alps.