Don't you mean he fought to FORCIBLY preserve the union? The South had enough of the "union" to the extent that they left to chart their OWN future. Why should this have mattered to your ancestor? He didn't live in the South at the time so it should not have impacted him in any way. What gave your ancestor the right to deny others their chosen path? He didn't care about slavery because it was legal at the time of secession. The truth is that your ancestor was ordered to fight on behalf of rich financiers in New York and Boston so they didn't have to risk their lives continuing the take advantage of the South charade and he was stupid enough to do so.
His ancestor might’ve been one of those poor ignorant Irish or German immigrants straight off the boat, my ancestors felt bad about having to shoot them, said those boys were being used as cannon fodder. It got so prevalent at one point they were convinced they were fighting Hessian mercenaries again, just like their grandfathers and great grandfathers had in the Revolution. Just another myth of The Great White North, I suppose. Why, of course we weren’t blowing illiterate Irish and German peasants to smithereens in our place in order to fight the war that our banksters demanded be fought.