Who was the first Roman emperor?

Augustus, and the point of his career is that he never used the word. Rome had expelled its kings five centuries earlier and the word rex was politically fatal; Caesar had been murdered partly for appearing to want it. Augustus took the title princeps, first citizen, held a collection of ordinary republican offices simultaneously, and formally restored the Republic in 27 BC while keeping control of the provinces where the legions were.

It worked because it let everyone pretend. The Senate continued to meet, elections continued to be held, and the constitutional forms were scrupulously observed while one man held the army, the treasury of Egypt and the power to veto anything. Historians call it the Principate, and it is the standard example of an autocracy built entirely out of existing legal machinery without a formal break.

His last words, reported by Suetonius, were to ask whether he had played his part well in the comedy of life and, if so, to applaud as he left. He had been born Octavius, was adopted posthumously by Caesar's will, won a civil war at nineteen, and ruled for forty-one years - long enough that almost nobody alive at his death could remember the Republic actually working.