Originally the word "volunteer" is of Latin origin: «voluntarius» ( «volunteer" or "interested person"). In Russian it is considered borrowed from the French «volontaire», meaning a person who voluntarily entered the military service.
As you may recall from history until the French Revolution in 1792 in the royal army consisted exclusively of mercenaries. French Commune was the cause of its collapse. But pretty soon it took the French military - to the state borders reached the Austrian troops. As a result, the male population voluntarily recorded in the ranks of the defenders of the homeland. That's when the word came into use. Peasants, workers and artisans of their own accord donned military coats, began to call volunteers. An interesting fact is that the soldiers themselves appoint a military commander. And in the case of dissatisfaction with his work, could be dismissed from service.