The magistrates who held imperium did not have full power inside the pomerium. They could have a citizen beaten, but not sentenced to death. This was symbolised by removing the axes from the fasces carried by the magistrate’s lictors. Only a dictator’s lictors could carry fasces containing axes inside the pomerium. […]
Provincial promagistrates and generals were forbidden from entering the pomerium, and resigned their imperium immediately upon crossing it (as it was the superlative form of the ban on armies entering Italy). Ceremonies of triumph, in which an army would march through the city in celebration of a victory, were an exception to this rule, although a general could only enter the city on the very day of his triumph, and would be required to wait outside the pomerium with his troops until that moment. Under the Republic, soldiers also lost their status when entering, becoming citizens: thus soldiers at their general’s triumph wore civilian dress.
Source: Pomerium – Wikipedia