Complex sentence: After fighting against Aspar's army, Aëtius managed to compromise with Galla Placidia; he sent back his Huns and obtained the rank of "comes et magister militum per Gallias", commander in chief of the Roman troops in Gaul.
Split into simpler sentences: After fighting against Aspar's army, Aëtius managed to compromise with Galla Placidia. He sent back his army of Huns and in return obtained the rank of "comes et magister militum per Gallias", the commander in chief of the Roman army in Gaul.