During the early tests of nuclear fission weapons in the deserts of the southwest in the 1940s, it was noticed that the extreme heat of the fission bLasts caused the silica of the sands to melt into a kind of greenish glassy substance. Little balls of the same glass like material are also a key indicator of the area of a big meteor or comet strike on the earth in the distant past.
Therefore, to "Glass" a planet means to hit it with weapons of such mass destructive force as to melt it down to a glassy slag, destroying all life and traces of civilization in the process.
Welcome to our universe of SE4. Only the strong survive here. Call it a kind of galactic social Darwinism if you will.
