It's not an t-arm because . . . of the leis?
That’s right. T-prothesis for masculine nominative singular nouns that begin with a vowel only happens when the noun follows an and it isn’t governed by a preposition.
+1 could anyone explain or link to the reason for this (or at least confirm is/isn't a mistake)?
Does this mean they are in the army and fighting, or that they are fighting against the army?
Like English, it could be either “alongside” or “against”.
Troideann na fir "san arm."