You need a level 3 city with a certain building in it. For empire, it's called War Council. For kingdom, it's called Command Post.

btw i really don't like these requirements. it makes sense to limit the quality of units by town size perhaps, but not their size. there are far too many individual units in the game, slowing down tactical combat already without forcing them on people in the late game. it would make much more sense to me to increase troop training times and make the garrisons more essential instead. but at the very least i don't think it's too much to ask to make war council a level 2 building, especially since people will end up getting this tech long before they have a level 3 city.