The end of Nero's reign, resulting from his extravagances and paranoid arrests, differed from the violent end of Caligula's reign in that there was no method of succession in place. While Claudius was certainly an unwanted choice by the Senate to replace Caligula, he did fill the role in a seamless transition that actually turned into a moderately successful reign. With Nero's suicide, knowing that the military revolts of his generals and legions were irreversible, the Principate faced its first dangerous challenge of civil war since the great wars that ended the Republic.

In AD 68, the revolt of Gaius Julius Vindex, governor of Gallia Lugdunensis was the final catalyst that brought the Julio-Claudian line to an end. He was joined in theory by the powerful...