"My" is an adjective, and in Latin it will have the same case, number, gender as the noun it describes.
In this sentence, "my" describes "comrades," whom the soldier hits: they are accusative, plural, masculine . So the corresponding form of the adj. meus, mea, meum is meos .
In some other sentence, perhaps it was "my father" (pater meus) who was doing something (as the subject of a verb): nominative, singular, masculine .
Hope this helps.