Following the header that introduced Helaman as the new record keeper, we have a verse that is certainly from Mormon. Mormon is pulling information from the large plates and creating a much smaller text. That means that there are times when he must create linking text to move from one story to the next, perhaps skipping a number of stories that were on the large plates, but which Mormon declined to add to his own book.
Verse 1 is just such a transitional narrative. Mormon told the story of the eighteenth year, and now moves to a new story. However, he finishes that previous story at the beginning of this chapter. Of course, the Nephites celebrated winning the war. Mormon describes the celebrations as having much fasting, praying, and worship. Doubtless there were more boisterous celebrations as well, but Mormon’s point is that God won the victory, and therefore the Nephites gave glory to God.