Simplify the following text for a Grade 12 native English-speaking person.
Original: At the head of a powerful army collected from Illyricum and the East , he entered Italy , determined to revenge the death of Severus and to punish the rebellious Romansp122 But due to the skill of Maximian , Galerius found every place hostile , fortified , and inaccessible ; and though he forced his way as far as Narni , within sixty miles of Rome , his control in Italy was confined to the narrow limits of his camp .
Simplified: At the head of a powerful army collected from Illyricum and the East , he entered Italy , determined to revenge the death of Severus and to punish the rebellious Romans . But due to the skill of Maximian , Galerius found every place hostile , fortified , and inaccessible ; and though he forced his way as far as Narni , within sixty miles of Rome , his control in Italy was confined to the narrow limits of his camp .