The first ten major generals appointed in the Union had six politicians among them. Also John A. Dix, George McClellan and David Hunter.
Those not on the list included three who would rise to stature in the war: Grant, Sherman and Sheridan.
As far as experience for general officers in commanding large groups, there wasn't much. Before the war (and after the Mexican War) the U.S. Army was involved primarily in fighting Indians and commands were small groups and companies. Much of the time was spent building forts.
Graduating classes from the U.S. Military Academy at West Point were placed according to what part of the class they graduated in. The top third became army engineers, Middle third went to the infantry and bottom third to the cavalry. A lot of your class standing at the Point came from your knowledge and speaking ability of French, which was THE language at the place. If you couldn't speak it, your chances of rising was not likely.
Duty on the frontier was boring and many officers out there took up drinking to pass the time.
Robert Toombs, Georgia politician and Confederate officer once said: "The Confederacy died of West Point."
--Old Secesh