Armies in minecraft is an organised group of people with weapons. The army in its moderm form was invented in the Exatian War 1. Young players who could have had a decent job and lead a good 1.1 century life with cats, were recruited, trained and sent to trenches instead. Coincidentally, the bow was invented around that time as well and so the states involved discovered, they can save on pensions for whole generations. They learn how to follow commands, which is clearly a useful skill in civilian life as well. In some countries, ex-soliders might have the opportunity to become teachers on fast track schemes and share the wisdom with the noobs.
While the army is typically an organised group of people with weapons, history also knows disorganised groups of people, also with weapons. Armies are typically state-run and there is one army per country. If there is more than one army in any one country, that's when it gets awful.
Given the state-of-the-art technology which exists today, it hopefully will not be necessary for soldiers to risk their lives, because drones will be sent instead. Noobs are used playing computer games, so they will be recruits with a highly valuable skillset.