I wouldn’t exactly call it an army... more like a horde.
Hordes are a viable means of conquest. Overwhelm the desired lands with motivated (i.e.: hungry, cold, angry/scared) people who have reason to settle and intent to not “assimilate”, people whose stakes are higher than current residents (”I’m comfortable & happy & want to be nice to others” vs “I want comfort & happiness & don’t care who I have to go thru to get my way”).