the war was not started to free the slaves
That is a true statement, however, in case anyone is of the opinion that slavery played no part in the manner in which the Civil War played out, one should take the time to read each seceding states Ordinance of Secession; one would thus be educated on the topic...
Let us say for the sake of argument that you are correct. Given that Slavery was legal and recognized by the Union as legal at the time, how does this justify an invasion into the land of people who want to rule themselves?
Speaking of education, if you actually read them, you will see that none of the original 7 and eventual 11 Ordinances of Secession mentioned slavery as a cause of their decision to leave the Union.
The lost cause losers are hair-splitting with you but I knew that you were referring to the various Declaration Cause documents put out by South Carolina and four other states: http://www.civilwar.org/education/history/primarysources/declarationofcauses.html
They all list slavery as their principle complaint.