In general, English, when a word ends with 'y' and includes a vowel, then to vary the word into its form, we replace 'y' with an 'i' and add es. the military could be a singular collective noun; hence isn't the word form of army. Armys isn't a dictionary word, and hence this selection is marked out.
Armies are the word form of army. It refers to multiple collective groups of people organized in an exceedingly military unit equipped for fighting. they're trained to serve their country during times of wars and peace. Both men and ladies is a component of it.
Example: the 2 armies were in position and would attack at the instruction of their senior.