National Guard units can be mobilized at any time by presidential order to supplement regular armed forces, and upon declaration of a state of emergency by the governor of the state in which they serve. Unlike Army Reserve members, National Guard members cannot be mobilized individually (except through voluntary transfers and Temporary DutY Assignments TDY), but only as part of their respective units. However, there has been a significant number of individual activations to support military operations (2001-?); the legality of this policy is a major issue within the National Guard.