Complex sentence: John Rodgers of the United States Navy had informed McClellan that he could not guarantee the safety of his army beyond the Landing, so McClellan had spent a good portion of the Seven Days Battles retreating to a place of relative safety.
Split into simpler sentences: John Rodgers of the United States Navy had informed McClellan that he could not guarantee the safety of his army beyond the Landing. So, McClellan had spent a good portion of the Seven Days Battles retreating to a place of relative safety.