Answer: The end of the Spanish-American War.

In February 1898, an attack upon the USS Maine, an American battleship that was then docked at Cuba, set off a four month conflict between the United States and Spain. The U. S. declared war on April 25th, and fighting had mostly ceased by mid August. The war came to an official end on December 10, 1898, with the signing of a treaty in Paris, France.

With the cessation of hostilities that summer, Philadelphia city officials decided to hold a Peace Jubilee celebration to commemorate the occasion. (Such events had been held before in other cities to mark of the end of, for example, the Civil War.) At the time, Philadelphia was the first city to hold a Peace Jubilee event for the Spanish-American War; as a result, it received national attention. The celebration lasted three days and included multiple festivities, such as military, naval, and civic parades.

HSP's J. Hampton Moore Peace Jubilee Celebration collection (#941) contains correspondence, photographs, circulars, lists, and other documents that are associated with Moore’s involvement with the event as finance secretary. Many of the photographs document the parade in progress and feature various military units and famous people, including President William McKinley.