The Union of Worlds was the government created by the Mule and led by himself as the First Citizen of the Union. During its height, the Union of Worlds encompassed a third of the known galaxy, including even Trantor and the core of the Galactic Empire. It was, militarily and politically, the greatest threat to the Foundation in history, due to its innate unpredictability.

After the death of the Mule, the Union of Worlds imploded. Under Lord Stettin it numbered at twenty-seven plants, but following the Stettinian War in 377 F.E. only Kalgan, its capital, remained under the Union's control.

Despite its dramatically fast gains, the Union's longevity was extremely brief - barely a decade between the Mule's initial war of conquest against the First Foundation and the civil wars that broke out upon the Mule's death in 306 FE. It gave a short glimmer of a polity that managed to re-unite a full third of the galaxy - a feat which was not to be matched by the First Foundation until two centuries later, around the time of the Eighth Seldon Crisis in 498 FE.