BUT WAIT!!!
That didn't mean the union were anti slavery.
That article writes
Sometimes, Loewen said, the North is mythologized as going to war to free the slaves. That's more bad history, Loewen said: "The North went to war to hold the union together."
Pres. Abraham Lincoln was personally against slavery, but in his first inaugural, he made it clear that placating the Southern states was more important. Quoting himself in other speeches, he said, "I have no purpose, directly or indirectly, to interfere with the institution of slavery in the States where it exists. I believe I have no lawful right to do so, and I have no inclination to do so."