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 376 F.E. only Kalgan, its capital, remained under the Union's control.