When the only crop is cherries, one must pick only that.
Attempts to make a cutesy quip are merely efforts to dodge the fact that the total evidence does not support the claim you are attempting to make.
Most soldiers did not write home about how passionate they were about stopping slavery. Those were the kooks. On the other side, most soldiers did not write home about how passionate they were about defending a nation that used slavery. Those were kooks on the other side.
Our FRiend Diogeneslamp argues that not every Northern soldier was motivated primarily by abolitionism, and not every Confederate fought to preserve slavery — both true enough and fair enough.
However...
First, historians have actually studied soldiers’ letters to see how many supported or opposed slavery, and, iirc, they were significant percentages.
Second, even in the Civil war’s earliest months, slavery was well understood on both sides to be critical to their efforts.
For Confederates slaves provided a huge workforce doing tasks otherwise requiring soldiers to do.
This slave workforce increased the effective Confederate army numbers by up to one-third — and that in turn reduced the Union Army’s overall manpower advantage from 2.5 to one to about 2 to one.
Such odds meant the Confederate army needed only to remain consistently on the defensive — i.e., behind baracades — to wear down & tire out the Union.
So slaves were critical militarily to Confederates but also personally to Confederate leaders, nearly all of whom were slave holders and so would suffer from abolition.
And precisely as vital as slaves were to Confederates, that’s how important emancipation was to Union leaders.
Even early in the war, every literate Northerner understood the Confederacy could not be fully defeated without emancipating Confederate “Contraband of war”.
So as Diogeneslamp does, to claim average civil war soldiers did not fight to abolish or preserve slavery is beside the larger point, which is that everyone on both sides understood its importance to the war’s outcome.
Lincoln of course understood the benefits of emancipation in Confederate states along with the necessity of preserving slavery in loyal Southern Union states.