> Those union soldiers were never fighting to end slavery. They were fighting to maintain the union.

The Union soldiers could and did fight for both things. I reminder that this one one of the most popular marching songs for Union troops at the time:

   John Brown's knapsack is strapped upon his back!
   His soul is marching on!
   His pet lambs will meet him on the way;
   They go marching on!
   They will hang Jeff Davis to a sour apple tree!
   As they march along!
   Now, three rousing cheers for the Union;
   As we are marching on!

Of course, individual opinions could and did vary a lot, but the very popularity of the song surely demonstrates the common sentiment among the troops. The elites, now, that is a more interesting question.