[gph]

Not technically the US, but the Confederacy did experience hyperinflation during the war. They ran the printing presses at full-tilt without anything tangible to back them up. And obviously once the tide of the war shifted things got real bad.

[nostrademons]

Also not technically the U.S, but during the Revolutionary War the continental dollar underwent hyperinflation, leading to the phrase "not worth a continental". By the end of the war they were worth less than 1% of face value and had ceased to circulate as money.