They are under the direction of each state’s governor, but could be mobilized into duty by the President under specific circumstances. In fact, many units were sent to Iraq and Afghanistan during those conflicts.

It’s unclear, though, whether the President has any ability to direct troops from one state to conduct activities in another. That may require the governor’s of both states to actually do the mobilization. However, there are a bunch of Republican states who would be happy exchanging troops to help arrest brown people.