The Union Leagues were quasi-secretive men's clubs established separately, starting in 1862, and continuing throughout the Civil War (1861–1865). The oldest Union League of America council member, an organization originally called "The League of Union Men", was formed in June 1862 in Pekin, Illinois. Four months later, on November 22, 1862, the Union League of Philadelphia, the first of the elite eastern Leagues and the second oldest ULA council member, was established.

Pekin is a city in and the county seat of Tazewell County in the U.S. state of Illinois. Located on the Illinois River, Pekin is the largest city of Tazewell County and the second most populous municipality of the Peoria metropolitan area, after Peoria itself. As of the 2020 census, its population is 31,731. A small portion of the city limits extend into Peoria County. It is a suburb of Peoria and is part of the Peoria Metropolitan Statistical Area.