Per Usamm.com:

While the National Guard prioritizes its state-level missions, there are instances where control shifts to the federal level. The President of the United States has the authority to federalize the National Guard under specific circumstances. This shift in control places Guard units under the same organizational chain of command as the active-duty military, making the Secretary of Defense their immediate overseer.

Federal control is most commonly exercised:

During wartime or military deployments to support overseas missions. To enforce federal laws wh****en local and state authorities cannot manage the situation (often requiring congressional approval). When there is a national emergency that exceeds the state's capacity to manage. For instance:

Following the September 11, 2001 attacks, National Guard forces were federalized to bolster homeland security efforts. Units were deployed internationally to support combat operations during the wars in Iraq and Afghanistan. Federalizing the National Guard dramatically changes its operational priorities, focusing entirely on national security over state-specific concerns. Who controls the National Guard at the federal level? The president of the United States mobilizes the National Guard, but the secretary of defense then controls them.