An anti-war movement is a social movement in opposition to one or more nations' decision to start or carry on an armed conflict. The term anti-war can also refer to pacifism, which is the opposition to all use of military force during conflicts, or to anti-war books, paintings, and other works of art. Some activists distinguish between anti-war movements and peace movements. Anti-war activists work through protest and other grassroots means to attempt to pressure a government (or governments) to put an end to a particular war or conflict or to prevent one from arising.

Substantial opposition to British war intervention in America led the British House of Commons on 27 February 1783 to vote against further war in America, paving the way for the Second Rockingham ministry and the Peace of Paris.

Substantial antiwar sentiment developed in the United States roughly between the end of the War of 1812 and the commencement of the Civil War in what is called the Antebellum era. A similar movement developed in England during the same period. The movement reflected both strict pacifist and more moderate non-interventionist positions. Many prominent intellectuals of the time, including Ralph Waldo Emerson, Henry David Thoreau (see Civil Disobedience) and William Ellery Channing contributed literary works against war. Other names associated with the movement include William Ladd, Noah Worcester, Thomas Cogswell Upham, and Asa Mahan. Many peace societies were formed throughout the United States, the most prominent of which being the American Peace Society. Numerous periodicals (such as The Advocate of Peace) and books were also produced. ^{[citation needed]}

A recurring theme in this movement was the call for the establishment of an international court to adjudicate disputes between nations. Another distinct feature of antebellum antiwar literature was the emphasis on how war contributed to a moral decline and brutalization of society in general. ^{[citation needed]}

A key event in the early history of the modern anti-war stance in literature and society was the American Civil War, where it culminated in the candidacy of George B. McClellan for US president as a Peace Democrat against incumbent President Abraham Lincoln. The outlines of the antiwar stance are seen: the argument of the costs of maintaining the present conflict not being worth the gains that can be made, the appeal to end the horrors of war, and the argument of war being waged for the profit of particular interests.

During the war, the New York Draft Riots were started as violent protests against Lincoln's Enrollment Act of Conscription to draft men to fight in the war. The outrage over conscription was augmented by the ability to "buy" one's way out, which could be afforded only by the wealthy.

After the war, The Red Badge of Courage described the chaos and sense of death which resulted from the changing style of combat: away from the set engagement, and towards two armies engaging in continuous battle over a wide area.^{[citation needed]}

William Thomas Stead formed an organization against the Second Boer War, the Stop the War Committee.