Why did some call the conflict a rich man's war but a poor man's fight?

In the South, it was mostly poor white trash fighting for the interests of big slave-owners.

In the North, a young draftee who could afford it, could pay another man (a substitute) to do his military service for him. This caused bad feelings in the ranks, and it was clear that these substitutes were mostly sub-standard rejects and bounty-jumpers who would never do any useful service in the front line.