Complex sentence: Domitian, devoting most of his army to attacking them near the Danube, called upon the Quadi and the Marcomanni to supply troops, they refused, and Rome declared war upon them as well.
Split into simpler sentences: Domitian, devoting most of his army to attacking them near the Danube, called upon the Quadi and the Marcomanni to supply troops. They refused, and Rome declared war upon them as well.