Why would the average Billy Yank take up arms and invade the Southern states? Either in unison or separation, he was not hostile to Southerners. Generally, the average person, Billy Yank or Johnny Reb, didn't hate somebody. The Billy Yanks were brainwashed that the South was populated by evil slaveholders. They were COERCED to hate their late countrymen. By telling them that the South was synonymous with slavery and was responsible for breaking the Union, they had controlled his mind and filled his heart with hatred and anger.
On the opposite end, the Johnny Reb rushed to defend his own country, his very state and home from an invader, a disturber of their peace and welfare. This gave them motivation enough to be a whip ready to punish the bull.
Civil War Scholar.