Many groups call themselves anti-war activists though their opinions may be different: some anti-war activists may be equally opposed to both sides' military campaign; in contrast, many modern activists are against only one side's (usually the most powerful) campaigns.
Pacifist and anti-war movements are similar, but not the same. Pacifism is the belief that violent conflict is never acceptable and that society should not be ready to fight in a conflict (see disarmament); the anti-war movement is not necessarily opposed to national defense. Pacifists oppose all war, but anti-war activists may be opposed to only a particular war or wars.
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 McClellan for President of the United States as a "Peace Democrat" against incumbent President Abraham Lincoln. The outlines of the anti-war stance are seen: the argument that the costs of maintaining the present conflict are not worth the gains which can be made, the appeal to end the horrors of war, and the argument that war is being waged for the profit of particular interests. During the war, the New York Draft Riots were started as violent protests against Abraham Lincoln's Enrollment Act of Conscription plan to draft men to fight in the war. 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.
On June 16, 1918, Eugene V. Debs made an anti-war speech and was arrested under the Espionage Act of 1917. He was convicted, sentenced to serve ten years in prison, but President Warren G. Harding commuted his sentence on December 25, 1921.
Veterans were still extremely cynical about the motivations for entering WWI, but many were willing to fight later in the Spanish Civil War, indicating that pacifism was not always the motivation. These trends were depicted in novels such as All Quiet on the Western Front, For Whom the Bell Tolls and Johnny Got His Gun.
Opposition to World War II was most vocal during its early period, and stronger still before it started while appeasement and isolationism were considered viable diplomatic options. Communist-led organizations, including veterans of the Spanish Civil War, opposed the war during the period of the Hitler-Stalin pact but then turned into hawks after Germany invaded the Soviet Union.
The war seemed, for a time, to set anti-war movements at a distinct social disadvantage; very few, mostly ardent pacifists, continued to argue against the war and its results at the time. However, the Cold War followed with the post-war realignment, and the opposition resumed. The grim realities of modern combat, and the nature of mechanized society insured that the anti-war viewpoint found presentation in Catch-22, Slaughterhouse-Five and The Tin Drum. This sentiment grew in strength as the Cold War seemed to present the situation of an unending series of conflicts, which were fought at terrible cost to the younger generations.
Opposition to U.S. involvement in the Vietnam War began slowly and in small numbers in 1964 on various college campuses in the United States. Countercultural works such as the notorious MacBird by Barbara Garson encouraged a spirit of nonconformism and anti-establishmentarianism. This anti-war sentiment developed during a time of unprecedented student activism reinforced in numbers by the demographically significant baby boomers, but grew to include a wide and varied cross-section of Americans from all walks of life. The anti-war movement is often considered to have been a major factor affecting America's involvement in the war itself. Many veterans of Vietnam, including U.S. Senator John Kerry, and disabled veteran Ron Kovic spoke out against the Vietnam conflict on their return to the United States.
The anti-war position gained renewed support and attention in the build up to the 2003 invasion of Iraq by the U.S. and its allies. Millions of people staged mass protests across the world in the immediate prelude to the invasion, and demonstrations and other forms of anti-war activism have continued throughout the occupation. The primary opposition within the U.S. to the continued occupation of Iraq has come from the grassroots. Opposition to the conflict, how it had been fought, and complications during the aftermath period divided public sentiment in the U.S., resulting in majority public opinion turning against the war for the first time in the spring of 2004, a turn which has held since. Anti-war groups protested during the 2008 Republican National Convention protests held in St. Paul, Minnesota in September 2008.
English poet Robert Southey's 1796 poem After Blenheim is an early modern example of anti-war literature — it was written generations after the Battle of Blenheim, but at a time when England was again at war with France. Alfred Tennyson's 1854 poem The Charge of the Light Brigade focussed on leadership failures and unnecessary loss of life in the Crimean War, while American author Stephen Crane's 1895 novel Red Badge of Courage, set during the American Civil War, cast war in a negative, anti-heroic light. World War I spawned the English war poets, such as Rupert Brooke, Wilfred Owen, and Siegfried Sassoon, as well as German novelist Erich Maria Remarque, all of whom (like Crane) used graphic realism to make a contrast with heroic notions of war.
Pablo Picasso's 1937 painting Guernica, on the other hand, used abstraction rather than realism to generate an emotional response to the loss of life from the fascist bombing of Guernica during the Spanish Civil War. American author Kurt Vonnegut used science fiction themes in his 1969 novel Slaughterhouse-Five, depicting the bombing of Dresden in World War II (which Vonnegut witnessed). The second half of the 20th century also witnessed a strong anti-war presence in other art forms, including anti-war music such as Eve of Destruction and One Tin Soldier and films such as M*A*S*H, opposing the Cold War in general, or specific conflicts such as the Vietnam War. The current American war in Iraq has also generated significant artistic anti-war works, including film maker Michael Moore's Fahrenheit 9/11, which holds the box-office record for documentary films, and Canadian musician Neil Young's 2006 album Living with War.