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

Answer 1

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.

Answer 2

Beliefs of people living in the Northern United States varied, though in general most believed that even if slavery should not be immediately banished, they felt that southern states had absolutely no right to secede and that the Union must be preserved. Also concerning slavery, even those who were not abolitionists, did not support slavery moving any further west because new slave states entering the Union would increase "slave power" and allow Southern States to control Congress. Additionally, the large portion of the Northern population made up of immigrants felt mainly indifferent about slavery but in any case did not want it to exist in the North because slave labor would cause competition with jobs that the poorly-paid immigrant factory workers would not be able to stand against.