The 1830 typesetter here changed the preposition by to with. Perhaps he felt that by seemed strange or that it might be misinterpreted as meaning ‘beside’ rather than ‘by means of ’. Or maybe his change was simply a typo.
Elsewhere in the Book of Mormon text, the preposition with is typically the one used with the verb guard:
The last example shows Oliver Cowdery, in his copywork from 𝓞 into 𝓟, miscopying the preposition with as by. Unlike the case here of Alma 27:24, in Alma 58:8 the 1830 typesetter maintained the reading in 𝓟 with its by. Undoubtedly the occurrence of the passive form of the verb guard made the use of by sound right (“which was guarded to us by an army of two thousand men”), whereas in Alma 27:24 the verb was in the active, which made the original use of by sound strange (“we will guard them … by our armies”).
The Oxford English Dictionary, under definition 30 for the preposition by, gives citations of by with the meaning ‘by means of ’, as in these two examples (spelling regularized), one from Early Modern English, the other from the 19th century:
In theory, either by or with will work in Alma 27:24 as well as in Alma 58:8. The critical text will, in each case, follow the earliest extant reading, thus by in Alma 27:24 and with in Alma 58:8.
Summary: Restore the preposition by in Alma 27:24 (“and we will guard them from their enemies by our armies”) and the preposition with in Alma 58:8 (“we did receive food which was guarded to us with an army of two thousand men”).