This is how Rome's Praetorian Guard held so much power
Few military units have ever had the effect on world history as did the Praetorian Guard. From the foundation of the Roman Empire until the reign of Constantine in 306, the Praetorians protected – and sometimes controlled – the leader of the most powerful empire on Earth.
Like other elite guard units to come, the Praetorian were loyal to the Emperor personally, not necessarily to Rome. At least, that's how it started under Caesar Augustus.
Over the centuries, the unit began to slowly corrupt. It soon became comprised of members of noble families who conspired against the Emperor, even assassinating a number of them.
"TAG! You're emperor now. Don't f*ck this up, Claudius. You saw what happened to the last guy who f*cked this up."
They weren't limited to the role of a mere guards.The Praetorian Guard fought in wars, in the Colosseum and other games, were a secret police force, and acted as volunteer firefighters for Rome. They would assist the regular firefighters in fighting larger fires.
After a number of assassinations, the Praetorian took their meddling in government a little too far. They were bound to butt heads with some Roman Emperor – without being able to kill him first. That emperor was Constantine I.
You don't get to be called "The Great" by staying at home. You get out and lift buildings with your bare hands.
The Praetorian Guard backed a pretender to the Roman throne. You can tell the pretender to a throne as opposed to the real Emperor because the pretender is usually filled with knives, spears, or poison.
Constantine defeated armies belonging to the General Licinius and Senator Maxentius and – unfortunately for the Praetorian – they backed Maxentius. Constantine liquidated and disbanded the Praetorian Guard, burned their barracks, and sent survivors to the far reaches of the Empire.