Complex sentence: First, he was as a commander of an invading foreign army on Italian soil, effectively cut off from the home country by a lack of seaborne resupply ability, his only hope of destroying Rome was by enlisting the support of her allies.
Split into simpler sentences: First, he was commander of an invading foreign army on Italian soil, effectively cut off from the home country by the difficulty of seaborne resupply. His only hope of destroying Rome was by enlisting the support of her allies.