Morel played a significant role in the British pacifist movement during the First World War, participating in the foundation of and becoming secretary of the Union of Democratic Control, at which point he broke with the Liberal Party. In 1917, he was jailed for six months for his antiwar activism, which had a permanent effect on his health. After the war, he edited the journal "Foreign Affairs", through which he sharply criticized what he considered French aggression and mistreatment of the defeated Central Powers. As part of his campaign against the French, he became the most important English proponent of the Black Shame campaign, which accused black French troops of outrages against the population of the occupied Rhineland.