system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How country borders are determined?

How are country borders decided to begin with and how are they enforced? What events decides/decided how large a country is/was?
gpt
A lot of factors come into play here - 

(1) Village/town/city boundaries can constitute the limits of a border. (Think Singapore)

(2) Geographic features and Rivers can often act as agreed upon borders.

(3) Historical military expansion and settlement can redraw borders. Some countries are even sometimes absorbed into larger ones, whilst collapse may cause a large nation to break into many smaller states (Such as Ukraine, Kazakhistan, etc breaking away from the USSR)

(4) Old Principalities or Kingdoms predicate modern nation states in some areas of the world.

(5) Treaties and agreements between states - often imperial powers - dictating where to draw the lines on the map (notice all the straight lines in Africa?). This also touches upon your second point of enforcement. Usually states have a map encompassing regions which they lay claim to and if it's an undisputed claim then it's recognized internationally through treaties and normal diplomatic relations. The problem comes when two countries lay claim to the same area at which point things become messy. (Think Kasmir between Pakistan and India). Countries either sort this out by holding diplomatic talks and hopefully resolve it peacefully, or they can go to war and occupy the disputed territory.