Complex sentence: Gen. Irvin McDowell, who was an inexperienced combat officer, but he commanded volunteer soldiers with even less experience--many of whom had enlisted for only 90 days, a period soon to expire.
Split into simpler sentences: Gen. Irvin McDowell, an inexperienced combat officer in command of volunteer soldiers with even less experience. Many of them had enlisted for only 90 days, a period soon to expire.