The original text here in Jacob 7:25 stated that the Nephites defended themselves “with their arms”, not “with their armies”. The 1830 typesetter accidentally misread arms (the reading in 𝓟) as armies. The original manuscript is not fully extant for the word arms, but the small lacuna between extant fragments allows room for the single final s of arms but not for the three letters ies at the end of armies:
The 1981 LDS edition restored the correct reading arms.
As Stan Larson has pointed out, armies would not have yet existed for the people of Nephi, given their small population at this time. The first references to Nephite and Lamanite armies are found in the Words of Mormon, which records events about 450 years after Lehi (and only after the Nephites had united with the people of Zarahemla):
(See page 251 of Stan Larson’s 1974 Brigham Young University master’s thesis, A Study of Some Textual Variations in the Book of Mormon Comparing the Original and the Printer’s Manuscripts and the 1830, the 1837, and the 1840 Editions.)
Summary: Maintain in Jacob 7:25 the original reading with its use of arms, not armies (“the people of Nephi did fortify against them with their arms”).