In the 19th century, faith in the rational, moral improvement of mankind, along with a revival of religious enthusiasm, spurred the peace movement. After the unpopular War of 1812, nonsectarian peace movements sprang up across the North, mostly appealing to well-educated white Protestants.

As the threat of war with the South grew, though, peace advocates struggled to define the limits of their stand. Were defensive wars permissible? Was peace that allowed terrible injustice worth keeping? And here the movement splintered. “O, yes — war is better than slavery,” wrote Angelina Grimké Weld, a political activist and strident peace advocate. The movement could not easily overcome the conflict between justice and peace — not then, and not a century later. Slavery had been abolished, but some 620,000 men in uniform had died.

The end of the war and the years of peace that followed, however, allowed many to put off the question. Late-19th-century Americans placed their faith in the progress of history. After the carnage of the Civil War and, in Europe, the Napoleonic wars, many believed that humanity had learned its lesson, and that world peace was a real possibility. Peace societies flourished. Activists formed international networks. A Swiss businessman established the Red Cross in 1863. At peace conferences at The Hague in 1899 and 1907, delegations established rules for neutrals and treatment of prisoners of war, and even an international arbitration court, in the hope of restraining warfare. (The most urgent reforms, like arms limitations and enforcement mechanisms — anything that might really limit state power — were off the table, but the conferences seemed a start.)

Money fueled the hope. In 1896 the inventor of dynamite died and left a will establishing the Nobel prizes, including one for peace. In 1910 Carnegie gave $10 million to found the Carnegie Endowment for International Peace. “Business pacifism” became a first principle of the Gilded Age. “Dead men buy no clothes,” said one industrialist in 1907.

But the difficulties of reconciling pacifist ideals with the reality of global politics remained. When world war came, most of the peace advocates in Europe and, eventually, the United States joined the fight, not because they were rejecting their own beliefs but because they were told repeatedly that it would be a war to end war. Only a tiny minority, including Evan Thomas (whose lifetime prison sentence was reduced to 25 years before he was released on a technicality), refused to fight.

Many would come to regret their support. Some retreated into isolationism. But others redoubled their efforts. International peace movements revived. Governments tried outlawing war (the 1928 Kellogg-Briand Pact is still on the books). Students held antiwar protests and signed pledges to refuse to fight. Peace advocates and statesmen wrestled to build the League of Nations. Time magazine chose Mohandas K. Gandhi man of the year in 1931. For a brief moment, pacifism seemed to be a driving force in international politics.