A Captain wrote in his letter back home that “if this war is to be fought over slaves, we couldn’t get two regiments to fight on the field”. This Captain was in the Illinois Infantry at Shiloh and referenced in a book about Shiloh.
I have read a quote from General Grant saying much the same thing, but others have disputed the authenticity of this quote. I don't know if it is true or not, but I have little doubt the sentiment expressed therein was true.