Please help me I don't understand history!

One problem that lead to the fall of the Roman Empire was its massive size. When a empire expands it borders, it need more soldiers to protect those borders. The Romans also introduced the concept of a standing Army, which was simply a army that stayed in a area, like a town or village, and protected it and neighboring areas. When Rome grew too big, they had to hire dis-loyal German Mercenaries. These mercenaries only worked to be payed, when Rome couldn`t afford to pay them, they quit. This lead to the Roman Borders being exposed. This allowed for Vikings and Barbarians to enter roman lands and later capture the city of Rome itself

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d: their own individual powers within the new goverment