The OTL George Thomas struggled with his decision between going with his native state of Virginia or to remain loyal to the Union. *In the end he chose the Union, influenced by his northern-born wife and his own antislavery sentiments.
He then went on to become one of the most gifted generals in the Union army in the ensuing Civil War, being one of the key contributing factors leading to the eventual Union triumph in the western theater.
What would have happened, what would be the butterflies, had be chosen to go with his state and family, and been commissioned a Confederate general, leading what became known as the Army of Tennessee?