Final answer:
Pertinax was the Roman Emperor assassinated by the Praetorian Guard after reigning for only three months, from January to March 193 A.D. His assassination highlighted the military's power over political stability in Rome.
Step-by-step explanation:
The Emperor who only ruled for three months from January to March 193 A.D., and was assassinated by the Praetorian Guard, is Pertinax.
Before becoming emperor, Pertinax had a career as a high-ranking military leader and senator. His attempts to restore discipline within the Praetorian Guard were met with resistance and ultimately led to his assassination.
During this period in Rome, the instability of leadership was profound, mostly due to the issue of succession and the overreaching power of the Praetorian Guard. This era, which followed the death of Commodus, is known as the Year of the Five Emperors, a time marked by rapid changes in rulership and political chaos.
The assassination of Pertinax exemplifies the volatile nature of Roman politics and the formidable influence of military forces like the Praetorian Guard during the twilight years of the Roman Empire.