Bush administration officials expect National Guard troops to begin deploying in June -- in support of the Border Patrol.
But authorities have yet to work out many of the key details of the immigration-related move.
The Department of Homeland Security today is to provide the Defense Department with a list of support missions for the Guard in California, Arizona, New Mexico and Texas.
Once that happens, the Pentagon will work with the individual states to match the mission requirements with available Guard units.
Permanently staffed coordination centers will be set up in each state to maintain continuity, but the field work will be done by units rotating in every two or three weeks.
The administration hasn't said how quickly the Guard force will reach the projected six-thousand-person mark.