What were some of the motives that led individual soldiers to fight in the civil war?

Answer 1

For the North, it was to restore the Union, and bring back the enormous cotton revenues.

For the South, it was emotive mission of supporting the rebellion, and defending their homeland against US troops who would now be seen as the invader.

On the Confederate side especially, many of the poor white trash were motivated simply by the prospect of wearing a uniform, carrying a rifle, and getting into a darn good fight.

Some people like to claim that it was over slavery. The poor white trash did not own slaves and were not too interested in fighting for the 'peculiar institution'. In the North, most of the rank-and-file were not so outraged about slavery that they would risk their lives fighting against it. If this were so, Lincoln would not have needed to issue the Emancipation Proclamation.

After the issue of the Proclamation, the destruction of slavery became the official war aim, but there is no evidence that new recruits rushed to enlist on account of this. (The mid-term elections of '62 did not show much new Abolitionist sentiment.)

And as you may remember from 'Gone with the Wind', when Rhett Butler joins the Army just when it was about to surrender, people may join armies for some pretty obscure reasons.