Sloan's growing discontent with what he called "the Plutocracy's government" led him to join the Socialist Party in 1910. Dolly Sloan also became active in Socialist projects at this time. John Sloan became the art editor of "The Masses" with the December 1912 issue and contributed powerful anti-war and anti-capitalist drawings to other socialist publications as well, such as the "Call" and "Coming Nation". As Sloan was never entirely comfortable with propaganda, his work for these magazines did not always contain overt political content. His belief that "The Masses" was becoming too doctrinaire led to a dispute with fellow editors Max Eastman and Floyd Dell, causing him to resign his position with that journal in 1916. He was never an ally of the Communist Party in the United States, although he remained hopeful that the Soviet Union would succeed in creating an egalitarian society. Throughout his life, he identified with left-wing political causes and expressed vehement disapproval of the inequities of the American economic system. A pacifist, he also opposed the American entry into World War I.