If you look at the causes the originally propelled the Union and the Confederacy to warfare it was the United States army fighting to preserve the Union and the Confederate States army fighting to protect slavery. Read the words of Lincoln, read the words of the secessionists in their state declarations of causes.
That's not to say that other causes were not the important factors to individual soldiers. Many reb soldiers fought not for slavery, but merely to combat a perceived external threat and there were Union soldiers whose goal from the beginning was to eradicate slavery.
“If you look at the causes the originally propelled the Union and the Confederacy to warfare it was the United States army fighting to preserve the Union and the Confederate States army fighting to protect slavery. Read the words of Lincoln, read the words of the secessionists in their state declarations of causes.”
Stop getting your history from Hollywood.
” My paramount object in this struggle is to save the Union, and is not either to save or to destroy slavery. If I could save the Union without freeing any slave I would do it, and if I could save it by freeing all the slaves I would do it; and if I could save it by freeing some and leaving others alone I would also do that.”
~Abe Lincoln August 22, 1862