Bankruptcy is a legal status, one that exists within the legal systems of many nations in various forms.

It does not really apply to sovereign nations themselves, however. I think you need to clarify what exactly you mean by "declare bankruptcy" in this question, if it is to be answered. What does that term mean to you in this context? Are you simply asking if the US has ever been unable to pay it's creditors?

So the question you want ask has the us ever defaulted on its debt.

There is two answers based on the definition used. Both are yes though.

The standard definition of default is being unable or unwilling to pay the debt due on the date it was due. That happened once. In April/may of 1979 there was a debt ceiling showdown in Congress like the ones you hear about so often and Congress passed a last minute approval. Unfortunately the late 70s technology meant, the large number of checks went out late and the US was technically in default. This caused rates to jump ~60 basis points.

The other definition we don't use any more. Basically the question is do currancy devaluations count as a default? Does reducing the value of a dollar from 1.6 grams to 1.5 grams count as a default? This is debatable and where they are probably where are drawing a bigger number from. The problem is the debt contacts, bonds are from understanding expressed in dollars, not ounces of gold/silver, so they aren't even technically a default.

Today this would be called currency manipulation, not a default or bankruptcy, if you did dropping the gold standard (Nixon shock) would be the ultimate one.

The number probably comes from Wiki which currently lists 6 events as US defaults. However those events are not defaults in the sense that the US could not pay back loans it had taken out.

The most recent event is the Nixon shock of 1971. In this event the US uniliterally ended the covertability of Dollars to gold, which till that time was the basis of the currency system. Which means on the one hand that the US did not pay back all the gold it has promised by vouchers (the US-Doller) to other nations. On the other hand everyone could keep their dollars and still use them to spent on products, however they lost value.

The question therefore is do you consider such actions of currency manipulation a default or not.