USD is really backed by guns.
The most realistic viewpoint I've seen, the only reason the USD has lasted so long is because of America's firepower, people have been too scared to stand up to them so they accept the U.S Dollar or face destruction, until Bitcoin came along of course.
Well actually the US dollar used to be linked to gold that's why countries and people were willing to accept it as the world reserve currency, you used to be able to get 1 ounce for 35 dollars I think, then the US started running perpetual deficits and people started wising up to the fact there's no way the US had enough gold to pay back the debt that it owed for running the deficits and that's when Nixon delinked the dollar to gold in '71. Since then we've been inflating bubble after bubble so the USD in it's current form hasn't been around for that long