During World War 1 Rees decided to dedicate herself to the war effort. Initially she worked on the New Zealand War Contingent Association's Entertainment Committee entertaining New Zealand soldiers in England; she gave out 20,000 tickets to New Zealand soldiers which she obtained from her contacts in the theatre world. By 1917 her health had deteriorated from overwork on the Entertainments Committee and, ordered to rest, she was offered a free trip to New Zealand by the military authorities. Instead, in early 1918, she joined one of Lena Ashwell's touring theatrical companies (the Lena Ashwell Dramatic Party), based in Rouen, entertaining the troops. She did this for 14 months; after the Armistice the Dramatic Party entertained the wounded at Ypres in Belgium and at Arras and Douai in Northern France.