This passage and the following four verses need to be repunctuated. The subordinate clause “when Moroni had armed all those prisoners” (identified by the arrow in the above citation) actually belongs with the following text. Only in verse 20 does Mormon return to the subject of Moroni arming the prisoners (“for he had armed those prisoners of the Nephites”); and then in verse 21 Mormon finally says what Moroni did after arming the prisoners: “and then he caused his men which were with him to withdraw a pace from them and surround the armies of the Lamanites”. In the middle of verse 17, the critical text will have a new sentence begin with the when- clause, while all the intervening text through verse 20 will be treated as parenthetical, thus stating in a Hebrew-like fashion, with an and before the main clause, that “when Moroni had armed all those prisoners … and then he caused his men which were with him to withdraw a pace from them and surround the armies of the Lamanites”.
The structural problem here in Alma 55:16–21 was first brought to my attention by Robert Baer (personal communication, 20 July 1989). Baer refers to an article by Larry Childs, in a FARMS Preliminary Report dating from 1986, “Epanalepsis in the Book of Mormon”, that more generally discusses this kind of repetition (sometimes called “resumptive repetition”) in the Book of Mormon text. (A summary of Larry Childs’ work in this area is found in the summer 1986 issue of Insights, the FARMS newsletter.) Here in Alma 55:16–21, we thus end up with two references to the arming of the prisoners, the initial one in verse 17 and the resumptive one in verse 20.
Summary: Restructure Alma 55:16–21 so that the subordinate clause “when Moroni had armed all those prisoners” in verse 17 is completed by the main clause in verse 21 (“and then he caused his men which were with him to withdraw a pace from them and surround the armies of the Lamanites”).