Rome existed for over 1000 years. As time went on they became more and more confident in their invincibility. They had the best military, technology, leadership, strategies, education, etc.
The dubbed their opponents "barbarians" insofar as the barbarians did not speak Latin. The barbarians were primitive, uneducated, and brutish. Their strength was in numbers and passion.
The fall of Rome did not happen overnight. It was a process. According to Edward Gibbon's History of the Decline and Fall of the Roman Empire, the following factors contributed:
Roman overconfidence & reduction of dedication
Accommodation of the Barbarians
Adoption of Christian ethics: non-violence, equality, justice, charity
DECISION: What is the biggest lesson to the U.S. and all nations from the example of the fall of the Roman Empire? How do empires die?