It is a Roman thing. At least them, probably most ancient civs had such a system. Everybody was either military or slave.
You're behind the power curve. Shoulda bought a copy, read it, and did your homework sooner.
http://www.google.com/search?hl=en&ie=ISO-8859-1&q=%22require+each+person+who+wishes+to+exert+control+over+the+state+to+wager+his+own+life%22&btnG=Google+Search
Copy/paste that string into Google... OR page 182-185 in the recent paperback edition (263 pages in length).
Check out the policy of mandatory service, and in fact the overall military philosophy, of Switzerland. A fascinating approach to civic responsibility, and the overwhelming majority of Swiss are on board with it.
As an aside, Switzerland is probably the scariest place for an army to try to invade, with the possible exception of the US.
Laz Long bump!