When Watson said rich mans war poor man's fight he was referring to?

This was being said on both sides.

In the South, a lot of poor whites felt they were fighting for rich planters, not in uniform, to continue their life of feudal privilege.

In the North, there was a bad law that allowed highly-placed young men to pay a substitute to enlist in their place. This not only caused much resentment; it flooded the armies with men of low ability and dubious character, supplied by unscrupulous brokers.

As for this Mr. Watson, I don't know who he was, and would be interested to know.