The main downfall of the Roman Empire was a tale as old as time.
The wealthy refused to pay their taxes and used their money to expand their wealth through the laws and the poor were too poor to pay them. Leading to collapsing vital infrastructure such as their cities, water, roads and roads bviously their military because you had to pay for your own equipment meaning a lot of the poor literally couldn’t afford to join the military to serve unless conscripted during emergencies which are no match for willing veterans.
Extra credits covered this quite a lot in the brothers gracchi segment.