What specific authority grants the President the power to federalize the National Guard for homeland security purposes?

The President's authority to federalize the National Guard stems primarily from Title 10 of the United States Code, specifically sections 12401-12406. These sections outline the circumstances under which the President can call the National Guard into federal service, which includes suppressing insurrections, enforcing federal laws, or responding to national emergencies. When federalized, the National Guard operates under the command and control of the President and the Department of Defense, rather than the state governors. This authority allows the President to deploy the National Guard to assist with homeland security missions, such as disaster response, border security, and civil unrest, when state resources are insufficient or federal intervention is necessary.