Once upon a time, a king named Offa ruled in the English kingdom of Mercia. Offa, who reigned c. 757-796, had a (relatively) huge empire, big enough so that Charlemagne treated him as an honored colleague. (Not as an equal, but then Charlemagne didn't treat anyone as an equal. Their friendship endured until Offa suggested that his son and heir marry one of Charlemagne's daughters, at which point Charlemagne was highly offended and broke off contact. But that's another story).

And here's Offa (he was rich and powerful enough to mint his own coins, some of which have survived with his portrait on them).

Late in Offa's reign, one of his money-makers designed him a new coin. On one side, it says "Offa Rex." And on the other, it says, "There is no God but Allah, and Muhammad is his prophet."

How did this happen?