Here initially in the printer’s manuscript, Oliver Cowdery omitted the subordinate conjunction when. Virtually immediately, he supralinearly inserted it (there is no change in the level of ink flow). Although the original manuscript is not extant here, there is clearly room for the when in the lacuna between extant fragments.
The main clause here is separated from the preceding when-clause by a long parenthetical present-participial clause (“both men women and children being armed with weapons of war / having shields and breastplates and headplates and being clothed after the manner of war”). Just before the main clause, the original text had the characteristic and that appears to represent a Hebraistic literalism. In this instance, John Gilbert, the 1830 typesetter, crossed out the ampersand in 𝓟 and ended up setting what we would expect in English. For further discussion of Gilbert’s tendency to delete such and ’s in his typesetting for the latter part of the text, see under Helaman 16:10. For a similar example of this kind of editing on his part, see nearby under Ether 15:30. The critical text will restore all these original instances of the Hebraistic and.
Summary: Restore in Ether 15:15 the original and that separated the when-clause (along with its associated present-participial clause) from the following main clause.