Why was Washington DC built where it is?

It was a deal. In 1790 the new federal government was deadlocked: Alexander Hamilton wanted the union to assume the states' war debts, which the southern states, having largely paid theirs, refused. Over a dinner arranged by Jefferson, the south agreed to the debt assumption and the north agreed that the permanent capital would sit on the Potomac, in the south. The city exists because of an accounting argument.

The site was swampy, undeveloped and chosen partly because it was nobody's existing city - a capital that belonged to no state, which is why the District has never been one. That has left its residents in a constitutional oddity: about 700,000 people, more than Wyoming or Vermont, with no voting representation in the Senate. The car number plates say taxation without representation, which is a pointed thing for a capital to print on its own vehicles.

The plan was drawn by Pierre L'Enfant, a French engineer who laid out grand diagonal avenues over a grid and was dismissed within a year for refusing to compromise with landowners. Much of the survey work was done by Andrew Ellicott and by Benjamin Banneker, a free Black astronomer and almanac-maker, who worked the celestial observations that fixed the district's boundary points.