[Sherman Logan]

The months of anarchy that followed ended with the ascension (usurpation) of Vespasian, the first of a number of military leaders to seize power in Roman history, and the final establishment of the precedent of dynastic rule.

You can't really call, IMO, the ascension of Vespasian a usurpation. This implies he took the throne from a "legitimate" emperor. Which again implies there is some well-known or accepted premise by which legitimacy is determined. Vespasian was the last of the "Four Emperors," and none of the other three had any real legitimacy either.

The Empire's biggest structural problem, IMO, is that they never came up with any such principle. Emperors came to power by all kinds of methods: inheritance, real or puppet election by the Senate, coup, civil war, etc. None of these was generally recognized as more legitimate than the others.

This had (at least) two great structural defects.

No Emperor could ever be sure of his generals' loyalty. He had to always be concerned about any competent general using his popularity to overthrow him. After all, that's how he or perhaps his dad got the job.

So emperors were always (and very logically) concerned about competent generals. Who they often executed or did not fully support. See Justinian and Belisarius. Although it is likely Belisarius was always loyal.

Meanwhile, overly successful generals often had a choice only between revolt or execution by the paranoid emperor.

OTOH, the primogeniture inheritance of the later European crowns, while it led to its own absurdities, allowed competent generals to be trusted by their monarchs, since they couldn't ascend the throne in any case. With the pretty much unique exception of Cromwell, and even he didn't call himself King. Louis XIV never had to worry that Turenne would displace him as King.

So Rome had civil war, over and over, every decade or two, through most of its imperial history. Think of what they could have accomplished with that energy turned outward, rather than inward, by a coherent rule of succession.

[Verginius Rufus]

Galba was the first usurper--unless you want to say Octavian was. But Galba turned out to be incompetent, and had no living sons. By Jan. 1 of the year after Nero's death, the legions on the Rhine refused to swear the customary oath to Galba but instead backed the commander on the Rhine, Aulus Vitellius. Meanwhile Otho had hoped to be adopted by Galba (thus in line to be the next emperor) but when Galba picked someone else with a more distinguished lineage (Piso), Otho engineered a coup which resulted in both of them (Galba and Piso) being murdered. Then Otho and Vitellius faced off--when Otho lost a battle to Vitellius' forces (Vitellius wasn't there), Otho committed suicide to prevent additional bloodshed. Vitellius quickly showed himself unsuited for the position of emperor, which is when Vespasian decided to make a bid for the throne.

The Year of the Four Emperors showed that anyone could become emperor if his supporters could beat the supporters of a rival, but they were fortunate to have no successful rebellions between 69 and 193. Vespasian was succeeded by his two sons, Titus and Domitian. After Domitian was murdered, the Senate put up Nerva, who quickly adopted Trajan. Trajan adopted his relative Hadrian (at least that was the official version). Hadrian adopted Antoninus Pius. Antoninus adopted Marcus Aurelius, but Marcus Aurelius had a surviving son who couldn't easily be passed over, Commodus. Commodus was another incompetent megalomaniac like Caligula and Nero and things went downhill from there.

The question is which Roman emperor the current President is most like...

Tacitus said of Galba, "in the opinion of everyone, he was capable of empire, if he had not ruled." I think of that in connection with some of the people who get hyped as potentially great Presidents if they would just run, like Colin Powell. He would have been a Galba.