Complex sentence: Arbogast had, based on his experiences fighting against the usurper Magnus Maximus in Gaul, decided that the best strategy was to keep his forces united to defend Italy itself, and to that end he went so far as to leave the Alpine passes unguarded.
Split into simpler sentences: Arbogast had, based on his experiences fighting against the usurper Magnus Maximus in Gaul, decided that the best strategy was to keep his forces united to defend Italy itself. He went so far as to leave the Alpine passes unguarded.