Between 260 and 274 CE, a series of generals ruled over the Gallic Empire. What about the Roman Empire, you are thinking? The Gallic Empire was a breakaway state that controlled the former (and future) Roman provinces of Germania, Gaul, Britannia, and for a time Hispania. It had five emperors in 14 years, printed it own coins, elected two consuls each year, and likely even had its own senate.
The Gallic Empire was a symptom of the Crisis of the 3rd Century, when Roman power was seriously challenged and breakaway states including the Gallic Empire and the Palmyrene Empires sprung up. Both were reconquered by the militarily capable Roman emperor Aurelian in 273 and 274, but the crisis did not really end until Diocletian took the purple in 284 CE.