While it is clear that the South fought the Civil War expressly for the defense of the institution of slavery, may it be said that the North was emphatically preoccupied with liberating the slaves as their chief goal? Or was eliminating slavery the North’s best way of stifling Southern power? I find it hard to believe in a time of such widespread racism—in the North and South—that the chief interest of Northern politicians in fighting the war was to liberate the people they continued to oppress well into the latter half of the 20th century. Since many of the issues leading up to the Civil War often involved whether a new state admitted to the Union would be a slave state or not, could the North’s motives in the Civil War be framed as a matter of asserting its federal dominance over the South?
The primary goal for the North was the preservation of the Union, from the very first day to the very last day of the war. There are countless examples of prominent political and military leaders in the North explicitly outlining this, and insisting that all else was subordinate to this, including the issue of slavery. While the vast majority of Northerners were against slavery in principle, and many in practice, this was not the primary motivation for most of them to engage in the war (except for die-hard abolitionists such as Frederick Douglass and William Lloyd Garrison). The origins of the Civil War were related to the potential expansion of slavery into western territories, not its current existence in Southern states. Some prominent Northern leaders, most notably Lincoln, initially assured the South that they were not intending on abolishing slavery in the South, merely curbing its westward expansion. The other leading Union figure, Ulysses S. Grant, also started out ambivalently about the issue of abolishing slavery and similarly was more concerned about preserving the Union than anything else.
However, as the war progressed views and agendas changed dramatically throughout much of the North, or at least in the circles of influential political figures. It was not uncommon to find references in personal correspondences to the abolition of slavery in the South being now just as important as preserving the Union. Both Lincoln and Grant, by 1863-4, insisted that the war would not end until slavery was abolished once and for all, and this sentiment was echoed throughout much of the army and in Washington among the Republican Party (most prominently by the Radical Republican segment of that party). Much of this change can be explained by both the witnessing first-hand of the injustices of slavery by the Union armies as they marched south, and also the positive contributions of black soldiers in the Union armies - Lincoln and several of his generals were adamant that black soldiers be allowed to fight, both out of a sense of justice and also to serve as a positive example that black people could be trusted and held as equals.
As always, it's impossible to make a single generalisation regarding what such a large part of a country thought, as there were and are always many different views at different times. But from all that I've read regarding the US Civil War and the sociopolitical context of it, it seems fairly clear that at the beginning of the war the primary motivation was not abolishing slavery in the South but preserving the Union, but that as the war progressed motivations and agendas shifted and these two issues came to be seen as deeply intertwined by many prominent political and military figures.
The Civil War was about slavery, full stop, although the war aims of the Union changed over time. For more, see our FAQ.