> The federal government needs a capital district it can defend militarily against the states. That's the whole point of D.C.
That overstates the matter. The federal government was incapable of defending the District militarily against the states, because it intentionally had no significant standing army at the time the District was ceded to it [0]. Even after the U.S. Army started to take shape — mainly for service on the frontier, not at the national capital — it's doubtful that the District could have resisted invasion by the militia of even one of its state neighbors; this is indirectly evidenced by the ease with which the British took and burned Washington in 1814.
[0] https://en.wikipedia.org/wiki/History_of_the_United_States_A...
[1] As noted below, a significant fraction of the African American population live in D.C. because their ancestors were brought here involuntarily. They should be permitted to vote.