This reminds me of something I've always wondered:
Why isn't the United States, unlike the Roman Empire before it, constantly plagued by ambitious generals swaying large fractions of the military to their cause and trying for ultimate power?
For the Romans, this was particularly a problem for legions stationed in far-flung corners of the empire like Syria and Britain.
So why do US forces in, say, South Korea or Germany never declare allegiance to their local commanders rather than the State?
Direct Deposit. Soldiers aren't paid by their commanders. The US had an problem early on when man wanted a commission he would be responsible for raising and outfitting his unit. The troops were very loyal to their commanders as they paid their wages, provided the gear and supplies they would need. There were cases of commanders disserting and their entire unit would leave having no other source of funds. The federal government eventually started paying the promised wages.
There were concerns about some elements of what you're talking about with some WW2 generals. For example, MacArthur was at one point considered too popular for it to be politically feasible to fire him, after his insubordination during the Korean War.
In general, I suspect this change stems from a few things. One is that generals do not lead soldiers into combat as directly any more. Another is that soldiers no longer stand to personally profit from combat or war -- there is no taking of booty, raping and pillaging are at the very least heavily frowned upon, etc. And another thing is that combat actions are less frequent these days, so any given soldier has a shorter sequence of experiences and victories with a given commander.
But I'm not an expert historian. These are just some guesses.
Even before the Roman Empire, all people in public life had to spend time in the military. In the Roman Republic you were elected to office, by a very limited voting pool, which included your military connections. It was thought of as more civil than the Roman Kings, who got to be in charge by right of military force.
The Empire grew out of the Dictatorship, which had been a title you were appointed to for a limited term. The term limits went out the window, and they never really established an orderly transition. It was ad hoc for four centuries. They had hardly two successions in a row decided the same way.
Americans, by contrast, prided themselves on their rules. They explicitly contrast themselves with the UK, which had a hereditary monarch. They pride themselves on their orderly democratic transitions.
It helps that George Washington voluntarily limited his term. I believe that his decision not to run again is one of the most important events in history. Americans have almost literally deified Washington; there's an incredibly embarrassing statue of him dressed as Zeus in the Smithsonian.
Washington was a military leader who respected the orderly transition, and that had been a huge part of American self-image. For a military leader to use the military to seize power would be almost unthinkable.
One reason is that civilians have ultimate authority over the armed forces, although generals have become President many times.
Another is that there is so much money to be made in the military and the surrounding defense contractors, and in the modern day glory with violence and conquering is less pleasurable than big houses and luxury cars.
Why isn't the United States, unlike the Roman Empire before it, constantly plagued by ambitious generals swaying large fractions of the military to their cause and trying for ultimate power?
For the Romans, this was particularly a problem for legions stationed in far-flung corners of the empire like Syria and Britain.
So why do US forces in, say, South Korea or Germany never declare allegiance to their local commanders rather than the State?