Forgive my ignorance. I’ve just become interested in the run up to the US Civil War. I just wondered, why did Lincoln and others have such fervour for preserving the Union at any cost? Was it not intended to be a union of freely associated states which meant that states could choose to withdraw if they wished?

Edit: I should say, of course abolition of slavery was something to fight for, but Lincoln seemed motivated by other factors as well, in fighting for the integrity of the Union? He was not originally wanting to force the abolition of slavery in the South, as wasn’t a Radical Republican - and considered the risk to the Union too great to attempt that? (I’m not sure why my question was downvoted, either?)