Every square that already produces a shield(s) produces an extra one. You can only build Militaristic thingies, Ie Barracks' and Harbours. The only way to go back to Nomalcy is by either making peace with all (or is it just one?) Civ(s), or by destroying the Civ you are at war with.
There is an exploit to do with Mobilization, but I refuse to say what it is. It ruins it... (not sure if it has been fixed yet)