The original manuscript has the word favored (spelled as favoured ) at both the beginning and ending of Alma 56:19. In the printer’s manuscript, Oliver Cowdery accidentally miscopied the second favoured as preserved (both words are visually similar). Here Helaman is simply repeating the idea that the Lord favored them by preventing the Lamanites from attacking. Note, however, that the repetition leads back to the decision of Ammoron not to attack the city of Judea but rather to maintain the cities which they had already taken (compare the end of verse 18 with the beginning of verse 20).
Elsewhere in the text, the verb preserve and the noun preservation are occasionally used to refer to the preservation of soldiers:
To be sure, the English language allows for armies to be preserved, so the secondary reading preserved is semantically possible in Alma 56:19.
The first occurrence of favored in verse 19 is, of course, probably not a mistake for preserved since elsewhere in the text “favored of the Lord” is fairly frequent (with six other occurrences) and there are no occurrences of “preserved of the Lord”. Moreover, we expect the preposition by with preserved (for instance, “preserved by the hand of the Lord” in 1 Nephi 5:14 or “preserved by the hand of God” in Alma 46:24).
Finally, we should note that there is evidence elsewhere in the text for beginning and ending a verse with identical summarizing phraseology, including the repetition of a thus, just as in Alma 56:19:
Summary: Restore the reading of the original manuscript in Alma 56:19 so that the ending as well as the beginning of the verse reads “thus were we favored”.