One of the reasons the Roman empire declined was the fact it just got too big, the republic couldn’t keep up with all that was happening within the empire and everything just got out of control because of the lack of authority throughout the empire. This automatically made the republic fall as well because the parts of the empire that didn’t want to be in the empire revolted and took back the land that the romans had taken. This even ended with the empire being split into 2 pieces in the end, and the western half feel into ruins even though Rome was in that half.
Please check my spelling and grammar if your gonna use this- hope it helps :)