David,

I believe the simplesst way to put it would be to compare it to today's Table of Organization. Modern organization dictates that:

2 or more companies constitute a battalion.
2 or more battlions constitute a regiment
2 or more regiments, a brigade ( with the exception of the USMC, which does not use the brigade as a standard organization as the Army does)
2 or more brigades a division
2 or more divisions a corps
2 or more corps an army
2 or more armies an army group

True that some battalions in the Civil War were autonomous but the War Dept. tried to alleviate this by combining battalions and coming up with a regiment whenever it was possible. During that time the use of the term "battalion", as an integral part of a regiment, was not common. Hence we have a cetain number of units with designated numbers for their individual ID. Today the forces still use this when the unit does not require a large enough number of personnel to constitute a full regiment. Today's battalions are as large as a Civil War regiment. A USMC Infantry battalion will contain from 900 to 1200 men depending on the situation. The Army, being top heavy, will have more in a unit.

Len