https://www.khanacademy.org/humanities/art-africa/east-africa2/ethiopia/a/the-kingdom-of-aksum

http://rightforeducation.org/2016/11/short-history-ethiopian-empire/

https://history.howstuffworks.com/historical-events/10-long-lived-empires4.htm

http://www.newworldencyclopedia.org/entry/Ethiopian_Empire