Alma 48:21 - 48:25
Alma 48: 21 But, as I have said, in the latter end of the nineteenth year, yea, notwithstanding their peace amongst themselves, they were compelled reluctantly to contend with their brethren, the Lamanites.
In spite of the peace that was had among the Nephites, they were compelled to go to war with the invading Lamanites at the end of the 19th year.
Alma 48: 22 Yea, and in fine, their wars never did cease for the space of many years with the Lamanites, notwithstanding their much reluctance.
This was the start of many years of war between the Lamanites and the Nephites.
Alma 48: 23 Now, they were sorry to take up arms against the Lamanites, because they did not delight in the shedding of blood; yea, and this was not all—they were sorry to be the means of sending so many of their brethren out of this world into an eternal world, unprepared to meet their God.
The Nephites were heart-broken over having to go to war but they were even more sorrowful knowing that the deaths from the battles would cause many to go to the eternal world unprepared to return to their God.
Alma 48: 24 Nevertheless, they could not suffer to lay down their lives, that their wives and their children should be massacred by the barbarous cruelty of those who were once their brethren, yea, and had dissented from their church, and had left them and had gone to destroy them by joining the Lamanites.
But the Nephites were not going to allow those were once lived among as their friends and neighbors and which had turned against them and join up with the Lamanites to attack their country, inflect death and injury on their families and friends.
Alma 48: 25 Yea, they could not bear that their brethren should rejoice over the blood of the Nephites, so long as there were any who should keep the commandments of God, for the promise of the Lord was, if they should keep his commandments they should prosper in the land.
The Nephites just could live with the thought that their enemies would rejoice over the killing of the Nephites and glory in it. This was not going to be allowed because the Nephites knew that if they kept the commandments they would prosper and be able to defeat their enemies.