Sallow basically selectively chooses elements from ancient Rome as convenient while discarding others, most clearly seen with his policy of ruthlessly stamping out the cultures of every people he conquers.

The Roman elements are just window dressing to the brutal, utilitarian dictatorship he's built to transform the wasteland. He's basically taken an idea and modified it to create something bizarre and alien and totally different from the societies of the wasteland.

Sallow's Legion not resembling Rome is kind of the point.

And surprisingly enough trying to run an entire civilisation like a rather brutal army doesn't work all that well. It is, after all, Caesar's Legion not Caesar's Empire.

Because Sallow didn't create a society based on the Roman Empire, he made a society based on the Roman Army.

While Sallow punishes even the smallest crimes with torture and death.

Furthermore Julius Caesar was extremely forgiving all things considered, and even favored using life imprisonment over capital punishment for crimes such as Treason. Yet Sallow despises democracies, and presumably denies his citizens a say in governance and presumably basic rights. Such as extending citizenship to all living in the Roman Republic, and Religious tolerance. And though many of his killers killed Caesar in the name of liberty and democracy, many of his reforms were making the Roman Republic far more liberal and democratic. The most glaring example being Slavery, the Legion heavily supports slavery, but Julius Caesar wished to decrease Romes dependency upon it. Hell many of Sallows practices are antithetical to that of Julius Caesar.

When you actually examine the Legion, it barely resembles the Roman Empire under Julius Caesar.

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