as in, "Today's ( = 今日, pronounced KYOU) border will vanish tomorrow, when The Man starts his next war." Ooh, political!!
If you stand on the earth over there, you can see all the way to the border
boundary - can be used with countries OR property lines of 2 pieces of land. Usually used when talking about crossing a boundary, or being within the boundary.
environment (like English, this is not limited to nature. A bad kid can be a "product of his environment.")
means border or boundary of property (80% the same usage as さかい （境） , but a bit more formal).
BOTH 境 and 境界 are often used in the figurative sense - like "This film pushes the envelope of good taste" or "His art blurs the boundary between painting and sculpture".
Also used in science.
The border is a line on EARTH but the mirror is made of METAL.
国境 境 境界 縁 端 末 先 外れ 際