Dropping my own favorite review of the contributing factors that led to the downfall of the republic and rise of empire:
"Storm before the storm".
"Storm before the storm".
Julius Caesar amassed an enormous military power with his private army on his military campaigns on Gaul and used it to just grab the power in Rome.
Truth, it was his nephew Octavian/Augustus that finished and buried the Republic, but he wouldn't be able to do so unless Caesar hadn't given the mortal blow before.
No wonder Putin tried to put a leash on Prigozhyn before it would be too late.
Also, Imran Khan must understand very well what happens when the military isn't under the government control.
In fact, I think he's quite similar to Philip II, who put all the pieces together in Macedon. Alexander the Great was able to reach such heights because he had the way paved for him, the same way Julius Caesar paved the way for Octavian.
This started with the Marian reforms.
Per a post a couple weeks back by the same author as TFA, those weren't a thing actually: https://acoup.blog/2023/06/30/collections-the-marian-reforms...
The Roman state authorised the raising of legions. But the (pro)consuls raising them were responsible for paying them. Think: Prighozhin.
That was fine for a few centuries, until external enemies re-emerged, and then it really fell apart.
The wealthy of Rome were not willing to pay for all of this, so the practice was for generals to pay their soldiers out of taxes and loot. Thus the legions became loyal to their generals instead of to the Republic, which essentially made them warlords. There were plenty of places left to conquer - Augustus would somewhat famously and disastrously try to conquer Germania east of the Rhine, Egypt was assimilated after Pompey's defeat, etc.
Interestingly, that disasterous campaign you mentioned by Augustus to try to conquer up to the Rhine was done exactly to address the difficulty of defending Rome's extended borders. He felt that it was important for the empire to achieve its natural borders, just the same as France sought to do after the revolution.
However, even had those natural borders been secured, it would have done nothing to address the internecine strife that was was instrumental to the decline and final collapse of the western empire. It is necessary to look at a combination of both internal and external pressures to explain what happened.