Alma 3:21 - 3:27

Alma 3:21 And it came to pass that there was an army sent to drive them out of their land.

A new army from Zarahemla was sent to meet the new threat for this latest invasion by the Lamanites.
It is interesting to note that all the battles in the Book of Mormon between the Nephites and the Lamanites were fought on Nephite lands. The Nephites did not invade Lamanite lands but a couple of times which was in spite of warnings not to do so which led to their destruction.

Alma 3:22 Now Alma himself being afflicted with a wound did not go up to battle at this time against the Lamanites;

Alma was not able to lead this Nephite force as he was recovering from wounds suffered from the fights with Amlicites and the Lamanites.
I think he was wounded in the one on one fight with Amlici and which he prayed for help to kill Amlici so he could continue to lead his people.

Alma 3:23 But he sent up a numerous army against them; and they went up and slew many of the Lamanites, and drove the remainder of them out of the borders of their land.

The Nephite army was a large force and they were able to soundly defeat the Lamanites and drive them out of Nephite territory.

Alma 3:24 And then they returned again and began to establish peace in the land, being troubled no more for a time with their enemies.

With the defeat of the Lamanite invaders and the death of Amlici, things were settling down to a normal existence at least for a time.
I am sure that everyone thought that the Lamanites would not leave them alone for long and they would be back.

Alma 3:25 Now all these things were done, yea, all these wars and contentions were commenced and ended in the fifth year of the reign of the judges.

All these events happened in the fifth year of the reign of the judges.

Alma 3:26 And in one year were thousands and tens of thousands of souls sent to the eternal world, that they might reap their rewards according to their works, whether they were good or whether they were bad, to reap eternal happiness or eternal misery, according to the spirit which they listed to obey, whether it be a good spirit or a bad one.

In this year the death toll from the fighting with the Amlicites and the Lamanites was in the tens of thousands. These dead were sent to the spirit world to be rewarded for their desires and their works in life. If they had tried to be righteous, they would be able to live in happiness for the eternities. If they, on the other hand, had not live as they should, their reward was misery and endless suffering. It was all contingent on who the individual chose to follow in their lives. If they listened to the prophets, they would be free of this misery, but if they listened and went along with self-serving people, then they would see their folly and suffer the pangs of conscience.

Alma 3:27 For every man receiveth wages of him whom he listeth to obey, and this according to the words of the spirit of prophecy; therefore let it be according to the truth. And thus endeth the fifth year of the reign of the judges.

Again, the rewards for the eternities were in relation to whom the individual choose to listen and follow in this life. The prophets and told us this from the beginning of time as well as been written about in the scriptures. In any case, this was the situation among the Nephites at the end of the fifth year of the reign of the judges.