In Peter Cozzens book Shenandoah 1862 he presents a much different picture of Banks than the one customarily seen. He reminds us that Banks was a popular general with his men and was one of the "self-made" men of the north and a regular rags to riches success. Banks, and every other Union general, made the error of assuming Jackson had abandoned the Valley after his defeat at Kernstown. Banks was left with a single division to hold the Upper Valley while everyone headed east to join McClellan/McDowell. Once Banks realized he was being outflanked by Jackson at Front Royal he abandoned the Upper Valley and hauled his division up to Winchester where they fought an effective rear guard action before recrossing into Maryland. Banks kept his command together and managed to keep them from being captured en masse (no small feat). Banks maintained the confidence of his men and then re-entered the Valley as one of Lincoln's Armies set on capturing Jackson.
Banks's war record is hardly shining apart from his ability to extricate himself from Jackson's grasp in 1862. But, hey, if you were part of that force he led out I am sure you were happy he was leading you that week! Banks gets beaten around at Cedar Mountain, plays no appreciable part at Second Manassas, and then heads west for more misadventures.
Overall a rather lackluster General forever to be clumped with Fremont, Shields, and Patterson.
Why was he promoted?
Come on now... he was a Republican! The high command of the Union was no place for soft-war Democrats
