Is such a force to be deployed domestically authorized under that "Constitution" thingy, or something? You know, involving Congressional appropriations, and all that.
There are multiple answers to your question.
President Trump federalized a million national guard troops.
The military can be brought in to work with government agencies in times of great peril to our nation.
When Continuity of Government is in place, there are a lot of things happening that you don’t see.