Gracchi at Westminster Wisdom comments on the transformational power for the state in changing from agrarian hoplite showdowns to the professional centurion:

It does that though at a cost - for the tribunes were right, what the state had done was ultimately to deprive some of the brute force that the populace had to withdraw its labour. It is much easier to withdraw your labour when your income does not depend on it - by financing wars through taxation and paying soldiers, the state had made the Roman population less capable of resisting its instructions.

Simple changes in organizational technologies can lead to disruptive advances in state power and politics. Arguably liberalism, constitutionalism and capitalism have made hyper-powers possible. (reply)
The Goths in the time of Constantine were heavily Romanised. The northern border of the Roman Empire on the Danube was quiet and peaceful. The Goths traded with the Romans and supplied mercenaries for the Roman Army. The Roman Empire by this time was Christian though the Goths were not. They still worshiped Pagan gods; however that was changing. Between the Goths capturing Christian Romans in prior warfare from the third century and integrating them into Gothic culture as slaves and peasants, and the increasing Romanisation of the Goths, Christianity began to be adopted. (more)
Now reading, The Day of the Barbarians. (more)
Cunning Realist writes on trauma cocktails which effectively make a nation accept anything; breaking down individual and social norms such that extremes become accepted as the new norm.

One of the interesting aspects of the Peloponnesian War was that the normal method of determining conflict between Greek city-states, hoplite battle, was replaced with political and ethnic genocide. Asymmetric warfare ruined the wealth, morality and power of Greece such that the Macedonians and then the Romans replaced them as the centre of Mediterranean power. (more)
It has only really been with liberal democracy that executive and martial power has been disentangled. During the reign of Darius and the establishment of the Persian Empire, the provincial tax collectors, or satraps as they were called, were given full military power. This mimics the imperium granted to pro-consuls and pro-praetors who acted as provincial governors and tax administrators (not tax collectors) for the Roman provinces in the middle and late republic. (reply)
Another data point for the definition of Empire being control of foreign policy. H.H. Scullard writes on Rome's run-in with Sparta:

But Flaminius would not go as far as his [Greek] allies desired; as with Phillip [of Macedon], he wished to cripple, but not destroy. Once again it was the Romans and not the allies who dictated terms, which included the surrender by Nabis of Argos and other towns and of his fleet, an indemnity, and the renouncing of the right to make war or alliances.
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Mos is one of the latin words the Roman used for constitutional tradition. One of the curious natures of the Roman Constitution and legal system was how unwritten it was. Much of it was ius , which me might call customary, to mos which Lintott describes as 'the way things were done at the time'. (more)

Hadrian is best known in Australia through Hadrian's wall in England which separated the Romans from the Celts of Scotland. Publius Aelius Hadrianus was the emperor of Rome between 117 and 138 AD. The emperors had been constantly changing the unwritten Roman Constitution in such a way to increase their power; by 138 AD there was no doubt that the emperor was sovereign. (more)

One of the most important surviving Roman documents is the lex de imperio vespasiani which was used to grant emperor's powers to Vespasian. It carries the words lex which suggests it was made statutory by an assembly, yet it is suspected that it is a Senatorial decree - consultum - that has been made into law. (more)

In his military campaigns into Gaul, Julius Caesar used Roman citizenship through service in the Legion as a means to introduce Roman property law; and more importantly displace Celtic tribal law. Citizenship became a mechanism for establishing Roman legal control in conquered Gaul. (more)
Cam Riley: South Sea Republic. Freedom, liberty, equity and an Australian Republic.