Complex sentence: For reasons unknown, Priscus did not pursue the Avars and aid Comentiolus, the latter having to evade to Iatrus, where his troops were routed nonetheless and fought their way south over the Balkan range.
Split into simpler sentences: For reasons unknown, Priscus did not pursue the Avars and aid Comentiolus. The latter was forced to evade to Iatrus, where his troops were nonetheless routed and had to fight their way south over the Balkan range.