I have discovered the Auto Garrison feature works for the defensive players huge advantage.
As the Union I discovered when we benifitted from Dallas Texas going Union, that I just kept the (2) Cavalry on the defense. The South attacked me and I got a free Garrison Unit so we stomped the attackers. Basically, let the South rampage the Deep West and the Union generates all the Defensive units we need. This is true throughout the whole West.
It works both ways however, if as the Union we are agressive and probe the South too much they generate all the defensive units they need, considering the South's limited manpower IRL this seems like a cheat they get so many "free" units.
As the Union I am willing to forgo the benifits of Auto Garrison; because as the South it skews the advantage way too much to their advantage as by nature the Union must be "Offensive".