Answer
Dara's fall to the Sassanids in 573 was a turning point in Justin II's reign. The loss of this strategically vital city, coupled with the emperor's pre-existing medical conditions and the stress of continual warfare, triggered a mental collapse that descended into full-blown insanity. Justin II's erratic behavior and impulsive decisions during this period left the Byzantine Empire in a precarious state, contributing to the challenges faced by his successors.