O Mula é parcialmente baseado em Tamerlane (também conhecido como Timur):
[My] notion of the Mule as someone who destroyed an apparently inevitable sweep of victory, which was then reconstituted after his passing, was based on Tamerlane’s disruption of the march of the Ottoman Empire—which resumed after Tamerlane’s death.
From a letter to Joseph F. Patrouch, Jr., published in The Science Fiction of Isaac Asimov (1974)
Bel Riose e O Cleon II não se baseia apenas em Belisarius e Justiniano I , como já mencionado pelo OP, mas também em Sejanus e Tibério :
Here’s a passage [...] from one of the Foundation stories, one which was originally called “The Dead Hand” and which was eventually included in Foundation and Empire, the middle book of the trilogy. It deals with Cleon II, Galactic Emperor, and while I wrote it, I had in mind Roman history. There was a little bit of Justinian and Belisarius of the sixth century and a little bit of Tiberius and Sejanus of the first century and so on.
From Opus 100 (1969)