One fact that most clearly shows the difficulty in defending the Roman Empire against invasion is the vastness of its borders. The Roman Empire stretched over a vast territory, covering approximately 2.2 million square miles (5.7 million square kilometers) at its height. This extensive border meant that there were numerous entry points into the empire, making it challenging to defend it effectively against invaders. The empire had to maintain a massive standing army to patrol and protect its borders, which required enormous resources and manpower. The sheer size of the Roman Empire made it vulnerable to attacks from multiple directions, increasing the difficulty in defending against invasion.
