The Militia is an paramilitary style organization that seems to have formed on The Border between the U.S. and Mexico after the apocalypse. It's command and control functions are unknown, but it can be assumed it has a structure similar to that of the U.S. Military. It's main function seems to be to protect the border and to prevent anyone crossing between the two countries.
Gallery
Nick and co being fired upon by the militia
Nick being 'detained' by a member of the militia
A member of the militia