Strategypage has an interesting overview of state defense forces.
These are the purely state-controlled military forces that exist in case a state's National Guard force is federalized to back up the active military.
State defense forces have various names and are military in name only for the most part, with potential for disaster response their most logical function. They could also perform local defense functions, at varying levels of competence.
But as the National Guard became more professional over the last century and became integrated with the active military (becoming an operational reserve to routinely support the active forces in the last decade), the prospect of a state losing its own military force has given these third tier forces a bit more profile. More significant for state needs are inter-state agreements to support each other with National Guard forces.
But these state defense forces are at least a framework for the states to rebuild their own militaries if that need seems to arise. One has to wonder if border states in the southwest might take the lead on this if illegal immigration reignites after our economy recovers.