Complex sentence: Apparently unbeknownst to McClellan, the Confederate Army of Northern Virginia was about evenly matched with the Federals, in terms of sheer numbers, possessing around 55,000 to 80,000 soldiers, and with Lee at the army's helm, the Confederates were notably more aggressive.
Split into simpler sentences: Apparently unbeknownst to McClellan, the Confederate Army of Northern Virginia was about evenly matched with the Federals, in terms of sheer numbers, possessing around 55,000 to 80,000 soldiers. And with Lee at the army's helm, the Confederates were notably more aggressive.