Complex sentence: Britannia became one of the most loyal provinces of the Empire until its decline, when Britannia's manpower started to be diverted by civil wars, eventually leading Emperor Honorius to order Roman troops back home to help fight the invading hordes.
Split into simpler sentences: Britannia became one of the most loyal provinces of the Empire until its decline, when Britannia's manpower was diverted by civil wars. Eventually Emperor Honorius ordered Roman troops back home to help fight the invading hordes.