It as my understanding that Sterling Price was a slaveholding Unionist, UNTIL he witnessed the "St. Louis Massacre" as the captured Camp Jackson soldiers were marched through St. Louis, by Fed. Gen. Nathaniel Lyon. Many of the conquering troops were German-born. The killing of soldiers and civilians disturbed Price so thoroughly, that he turned from his Union leanings to the Confederacy.
Some feel that Union Gen. Lyon purposely stirred the already heated pot.
Gen. Sherman and his son were also by-standing witnesses at the "massacre".