When world war broke out in 1914, Dennett to join the Women's Peace Party, an anti-war movement. In 1916, she served as field secretary for the American Union against Militarism, organizing meetings in several large cities. Dennett's work to re-elect Woodrow Wilson (under the belief that he would not declare war) led to a respected job as executive secretary for the League for Progressive Democracy. She resigned after Wilson declared war in 1917. She next co-founded and was employed by the People's Council of America, a socialist peace movement inspired by the Bolsheviks.