re: Is Washington DC a part of the United States?

It's a federal city designated as the Capital of the United States of America by the Residence Act of 1790 which is a duly enacted piece of legislation by the duly elected Congress of The United States of America composed of representatives from the states themselves and Congress is a body as duly established by the Supreme Law of the Land and that's the Constitution of the United States of America which was ratified by the 13 original colonies that became states. Can't get more American than that.

Not sure what you're smoking tonight Mogray but it has to pretty good if you're thinking this hard.