Here 𝓞 reads “in the defense of their country” (with defense spelled as defence). The RLDS editions lack the definite article the, reading “in defense of their country” (in accord with expected English usage). Elsewhere in the text, we have seven occurrences of “in the defense of X”, but only one of “in defense of X” (marked below with an asterisk):
For two of these examples, there is evidence for the loss of the the:
The last example shows the momentary loss of the the when Oliver Cowdery copied the text from 𝓞 into 𝓟, which therefore implies that the one manuscript example without the the (in Alma 51:20) could be an error, even though that example is extant in 𝓞 and shows no sign of the the, not even a weakly inserted supralinearly the.
Moreover, the phraseology in Alma 51:20 (“in defense of their country”) is otherwise identical to two other cases, in Alma 61:6 and Alma 62:9, and is similar in all other cases. The most consistent solution, then, would be to assume that Alma 51:20 is actually a scribal error and that the text there should be emended to read “in the defense of their country”. Nonetheless, “in defense” is possible in English, so we may simply have a unique occurrence without the the in Alma 51:20. For that one case, the critical text will therefore retain “in defense of their country”, the reading of all the textual sources (including 𝓞).
In today’s English we expect “in defense of X” rather than the now archaic “in the defense of X”. Evidence for the latter can be found throughout the history of the English language, from late Middle English on, as in the following examples gleaned from Literature Online (given here with original spellings and capitalization):
Summary: Follow the reading of the earliest textual sources concerning the question of whether the should occur in the phrase “in (the) defense of X”; in eight cases, we have the the, but in Alma 51:20 the the appears to be lacking; this lack of the may nonetheless be an early error that entered the text since there is textual evidence for the tendency to omit the the from that phrase (momentarily in 𝓟 for Alma 62:9 and twice in the 1874 RLDS edition for Alma 53:13 and Alma 62:5).