Answer:
because they did not like each other
Explanation:
Im pretty sure the answer is c if not then b
A. Emperor Alexander Severus, who was assassinated by his troops after a defeat.
B. Roman Senade, which accepted some army generals as legitimate emperors.
C. Security of the border, which they were supposed to keep. While the were fighting among themselves, they neglected the defenses of the Emprire, resulting in invasions.
D. Crisis, which ended with the Reforms made by Diocletian.
