Complex sentence: This honour dates back to ancient Rome which regarded the "Pomerium", the boundary of the city, as sacred and promagistrates and generals were forbidden from entering it, and resigned their "imperium" immediately upon crossing it.
Split into simpler sentences: This honour dates back to ancient Rome which regarded the "pomerium", the boundary of the city, as sacred. Promagistrates and generals were forbidden from entering it, and resigned their "imperium" immediately upon crossing it.