• The National Guard is both a branch of the military and a state militia at the same time. This means that the primary duty of the National Guard is to defend the United States domestically and to assist in emergencies, but in many cases the Guard can also be used by the federal government. What the rules for deployment are in any given circumstance will vary, but here are some solid examples.
  
  

State Missions

• When the National Guard is activated to help with a state mission, such as assisting in natural disasters like Hurricane Katrina, the deployment tends to be for anywhere from 15 to 60 days. The state executive government (headed by the governor) will typically call up the Guard to deal with an emergency. Each state National Guard has its own hierarchy, but if Guardsmen from more than one state are assisting, then there will be a joint effort.
  
  

Federal Missions

• When the federal government activates the National Guard, the Guardsmen change status from state militia to federal soldiers. Typically, the federal government only activates the Guard in times of domestic need, or when overseas operations need to be reinforced. Federal deployments of the National Guard tend to last 12 months, and these deployments must be authorized by Congress. This is not to say that each individual deployment is authorized, but rather that Congress agrees that the National Guard should be activated and used by the executive branch and by the military.
  
  

Active Duty Assignments

• Members of the National Guard may volunteer for a number of active duty assignments. According to nationalguard.com, one of the assignments that Guardsmen may volunteer for is border patrol. These assignments are on a volunteer basis, so there isn't a general set of rules to govern when they can happen. Generally speaking, the commanding authority will lay out what the Guardsman is volunteering for, how long he'll be deployed and what sort of compensation he can expect.