President Wilson called for a war resolution on the evening Congress convened in April 1917. A strong advocate of women's rights, she was lobbied by rival women's political groups tried to persuade Rankin that her vote would speak for all women. Carrie Chapman Catt of the National American Woman Suffrage Association feared a vote against war would brand suffragists as unpatriotic while Alice Paul of the Woman's Party thought women in politics should speak for peace. Rankin, who had not previously identified herself as a pacifist, announced that she could not vote for war and in joining the fifty-six other Members who voted against the war resolution embarked on the cause that would be at the center of her life until her death more than a half century later. Her vote cost her reelection.
For twenty years following her retirement from Congress in 1919 Rankin was actively involved in a variety of pacifist organizations such as the Women's International League for Peace and Freedom. She served also as field representative for the National Council for the Prevention of War.
She was elected to the House of Representatives again in 1940 on a peace platform and was the only member of Congress to vote "no" to the U.S. entering the second World War. To vote "no" to war after the Japanese had bombed Pearl Harbor took great strength of conviction. After placing her vote, Rankin had to seek safety in a nearby phone booth from angry crowds until police could escort her home.
In the years following the war, Rankin made various trips to India to study the pacifism of Gandhi and others. She briefly reentered public life in the late 1960s when a coalition of women organized themselves as the Jeannette Rankin Brigade and marched on Washington in protest of the war in Vietnam.