The purpose of the National Guard is to defend the United States at home and abroad. We can support the defense of the homeland by being activated by the Governor for Domestic Operations, like for natural disasters, or for attacks carried out in the US, or we can be activated by the President for defense of US interests overseas like in Operation Iraqi Freedom/Operation Enduring Freedom (Afghanistan) and many other Overseas Contingency Operations. Your National Guard has been involved in protecting the citizens of the US since the Revolutionary War. This is different from the active Duty in many ways. You would have to contribute 1 weekend per month and two weeks per year to military training, unlike the active duty which is in military status all the time, and the active duty cannot be activated for long periods of time in the US due to constitutional law and other legalities. To break it down to the intent, the constitution states that the President may not us the armed forces in the US (Marshall Law) because the founding fathers were trying to protect the citizens from imperialistic tendencies by the President. Basically, they didn't want the President to be able to use the military like a King would, which is why Congress approves mobilizations. So, the active duty is used for overseas operations, and the National Guard serves in Domestic and Overseas operations.