>>512209979
Most of it was principle. The constitution was a treaty between the British colonies, in the treaty it codified slavery. Every time a new free state was added a slave state was added. This was the balance. The north never once attempted to peacefully prevent slavery in the West, instead they stole all the territory, territory which was won by southerners like Polk. Instead of slavery being left to die in a hole the south had heavily industrialized and figured it could make it on its own. The north needed the south's tax base and output to accomplish their goals of world domination so they invaded. Simple as