Letters of typical Federal soldiers, like those quoted by McPherson, read much the same. They're fighting to preserve the authority of the government. They use almost exactly the same words. Not as many mention emancipation or slavery.
You mentioned states rights. Based on message board discussions over the past few weeks, I'm curious to know what you believe that to mean to Southerners 150 years ago. For what it's worth, I don't recall seeing SR cited by a Confederate soldier in a letter or diary written during the war.