Why did we have the American Civil War and what does it mean as said in DC 87? Will those people be saved due to fighting for their country? What about temple work for them?
The reason for the American Civil War was fundamentally the same as the reason for all wars: the failure of humans to live in accordance with the principles of the Gospel of Jesus Christ. At the beginning of the Iraq War in 2003, President Gordon B. Hinckley offered a conference sermon regarding the Church’s stance on war and responsibilities of Church members in wartime. I believe that, generally speaking, the principles President Hinckley outlined were as true at the time of the Civil War as they are now.
As for the salvation of those involved in that or other wars: The Lord has instructed us to do temple work for as many as possible and, to my knowledge, has made no provision for barring temple work on the basis of participation in any particular armed conflict. For these individuals, as with the rest of us, “the Lord’s way of final judgment will be to apply His perfect knowledge of the law a person has received and to judge on the basis of that person’s circumstances, motives, and actions throughout his or her entire life” (Dallin H. Oaks, “Judge Not” and Judging).