The beginning of verse 15 underscores how the Book of Mormon differs from more modern cultural expectations. It begins as might be expected with the taking of prisoners. The difference is where they were taken.
Modern assumptions would have captives taken back to the land of the victors. These prisoners are taken back to their city. Modern assumptions would have the captives imprisoned, but these are returned to their lands and lives. Modern assumptions would have the victors looting the city and taking anything of value, but these captives surrender half. That is clearly a large amount, but significantly different from losing everything.
Each of these actions differs from modern expectations but are perfectly in line with common practice in Mesoamerica. The purpose of conquest in Mesoamerica was to set up lines of tribute. The genius of that system was that the conquerors received goods not only immediately after the conquest but created an ongoing stream of goods. Leaving the people in place allowed for the continued creation of the goods and redirected a percentage of them from the inhabitants to the victors.