What did the North side believe in during the Civil War?

The North believed that the Southern states had no right to quit the USA. And they wanted the cotton revenues back.

When Lincoln issued the Emancipation Proclamation (effective from January 1863), he officially turned the war into a crusade against slavery. This was totally successful in deterring Britain from aiding the Confederates, as it would have made them look pro-slavery themselves.

But it did not convert many neutral Northerners into Abolitionists, and the average view of the war had not greatly changed.