The Constitution repeatedly refers to the “Militia,” which under the Constitution is supposed to be a state-based military force consisting of part-time soldiers used exclusively for state purposes—unless the President “calls them forth” to enforce federal law, repel invasion, or suppress insurrection.
But during the 20th century, the federal government renamed the militia the “National Guard.” More seriously, it seized control of it from the states and began using it for all sorts of purposes as a kind of “military on the cheap.” As a result, when states need their own guardsmen/militiamen for fighting natural disaster or for civil unrest, they often find that they are unavailable because the feds have deployed them to places like Syria and Iraq—in complete defiance of the Constitution.
Rob Natelson and co-author Liam McCollum, have written an article summarizing the Constitution’s rules, how they are being violated, and what to do about it. It is now under consideration by scholarly law journals. You can find a draft of the article here.