The League to Enforce Peace was a non-state American organization established in 1915 to promote the formation of an international body for world peace. It was formed at Independence Hall in Philadelphia by American citizens concerned by the outbreak of World War I in Europe. Support for the league dissolved and it ceased operations by 1923.

In 1905, Theodore Roosevelt, often in coordination with Republican leaders Henry Cabot Lodge and William Howard Taft, began offering proposals for the formation of a League of Nations to advance world peace. In his 1905 annual message to Congress, Roosevelt identified the need for some method of control of offending nations, which would ultimately become the responsibility of an international body.

In his Nobel Prize acceptance address, Roosevelt said: "it would be a master stroke if those great Powers honestly bent on peace would form a League of Peace, not only to keep the peace among themselves, but to prevent, by force if necessary, its being broken by others." The planned league would have executive power such as the Hague Conventions of 1899 and 1907 lacked. He called for American participation.

Few expected a world war to start in 1914, but the July Crisis launched World War I and elevated the cause of peace to an immediate concern. Activism calling for the formation of an international organization to contain and respond to violence began in 1914 with speaking tours.

In September 1914, Roosevelt proposed "a World League for the Peace of Righteousness," which would preserve sovereignty but limit armaments and require arbitration and added that it should be "solemnly covenanted that if any nations refused to abide by the decisions of such a court, then others draw the sword in behalf of peace and justice." In 1915, he outlined this plan more specifically, urging that nations guarantee their entire military force, if necessary, against any nation that refused to carry out arbitration decrees or violated rights of other nations. He insisted upon the participation of the United States as one of the "joint guarantors." Roosevelt referred to this plan in a 1918 speech as "the most feasible for...a league of nations."

By this time, Wilson was strongly hostile to Roosevelt and Henry Cabot Lodge, and developed his own plans for a rather different League of Nations. It became reality along Wilson's lines at the Paris Peace Conference in 1919.

Advocates worked to distinguish their efforts from antiwar efforts aimed at preventing American participation in the war and to counter misimpression that they were trying to end the war in Europe. Hamilton Holt published an editorial in his New York City weekly magazine The Independent, "The Way to Disarm: A Practical Proposal", on September 28, 1914, which called for an international organization to agree upon the arbitration of disputes and to guarantee the territorial integrity of its members by maintaining military forces sufficient to defeat those of any non-member. The ensuing debate among prominent internationalists modified Holt's plan to align it more closely with proposals offered in Great Britain by Viscount James Bryce, a former British ambassador to the United States.

At a convention at Independence Hall in Philadelphia, on June 17, 1915, with former U.S. President William Howard Taft, serving as the league's first president, presiding, 100 notable Americans formally announced the League to Enforce Peace's formation. They proposed an international agreement in which participating nations would agree to "jointly use their economic and military force against any one of their number that goes to war or commits acts of hostility against another." The league's founders included Elihu Root, Alexander Graham Bell, Rabbi Stephen S. Wise, James Cardinal Gibbons of Baltimore, and Edward Filene on behalf of the recently founded U.S. Chamber of Commerce. Elected to the executive committee were Harvard University president Abbott Lawrence Lowell, former Cabinet member and diplomat Oscar S. Straus, magazine editor Hamilton Holt, Taft, and a dozen others