What weakened many of the ancient cities leaving them vulnerable to nomads?

Alot of things weakened ancient cities, whether it be lack of natural barriers to defend against impending invasions or So much territories they couldn't  govern! The salinization of soils,Which made soil saltier which produces abundances of food that feed soldiers, if this is missing,this will make them less willing to fight. The civilization could be surrounded by mountains making it easier for any enemy to pounce on the city! Hoped this helped ! ~ Feel free to mark brainliest ~

it was on the rise and doing very well and would continue to do well until the great depression from 1929-1939

lol   that was   funny : >