You could argue that the North was not fighting against slavery. As slavery in the Confederacy was only abolished with the emancipation proclamation and slavery in the Union with the 13th Amendment. The were fighting because of slavery, as disagreements over the spread of the institution led more or less directly to the secession, but they were not initially fighting to abolish it.

Arguing that the South was not fighting for slavery, however, is disingenuous. The conflict over territories becoming free or slave states upon admission was the biggest issue of argument between the North and South.