The US government has confiscated value from T Bills before.

The government used to sell two bond types - dollar bonds that paid off in dollars, and gold bonds that paid off in gold. The gold bonds were safer and hence paid a lower interest rate.

Enter FDR. FDR decided to pay off the gold bond holders in dollars, not gold, and since the value of gold vs dollars had diverged substantially, FDR confiscated the difference.

That was the end of the phrase "sound as a dollar". Gee, I wonder why nobody says that anymore!

The largest risk of TBills is that inflation will shrink their value, and with catastrophic deficits that is a very, very real risk. That's why I don't invest in bonds or any investment that is denoted in dollars.