During the third century the imperial court became more militarized than ever. The emperors were almost constantly in the field with their troops and almost always in uniform. Hence "the transformation of the princeps into a pure imperator, and of palace into camp (castra)." This development naturally affected the character of the emperor's staff. It ceased to be dominated by freedmen and chamberlains; the tone was set instead by men drawn from the officers corps. At the same time the emperors sought to organize new military units on which they could depend for security. Gradually these two tendencies converged.

Generally our sources throw light on the emperor's security arrangements only in connection with their assassinations. We can best review developments, therefore, by examining a few of the better-documented plots of the third century.

Caracalla was cut down while campaigning in Mesopotamia. He had left his army to visit a temple, and was attended only by a small escort which included his bodyguards. The latter, whome Caracalla called his "lion," were true corporis custodes, as is clear from Dio's description: "For the emperor kept Scythians and Germans about him, freemen and slaves alike, whom he had taken away from their masters and wives and had armed, apparently placing more confidence in them than in the soldiers." Caracalla's immediate staff, however, was composed of regular army officers: Decius Triccianus, prefect of the Legio II Parthica and the commander of the escort, two praetorian tribunes, and an NCO serving as groom. This was the key to the situation. All four officers were in league with the praetorian prefect, and it was a simple matter of wating for the right moment to assassinate the Emperor.

Caracalla's removal marked the triumph of the general staff, the emperor's officiales. These were "new men," not part of the senatorial aristocracy. The prefect Macrinus became emperor, the first equestrian to gain the supreme power. Triccianus, who had begun his career as orderly to the governor of Pannonia, now returned to Pannonia as governor himself. And the post of urban prefect, traditionally given to a leader of the senatorial order, went to a man who had risen from the ranks in the military intelligence service. A new class began to take power.

R.I. Frank, Scholae Palatinae: The Palace Guards of the Later Roman Empire, Papers & Monographs of the American Academy in Rome Vol. 23. (Irvine, CA: American Academy in Rome, 1969), 30-32.