The Liberty Loan was used to finance World War I. United States Treasury bonds are a relic of the act of congress at issuing these loans. Overall there were 4 issues of Liberty Loans as well as a Victory Loan. The first and second issues occurred in 1918 and the third and fourth occurred in 1918 (Source: Wikipedia). A much scarcer variety from 1917 also exists (Source: Heritage).
Shown below is a $1 coupon from the Second Liberty Loan of WWI.
$1 Bond Coupon, United States Of America (1917)
Payable May, 1919
Image Contribution: Handini
Although this is a United States issue, and none of the currency issued by the United States of America for circulation since 1861 are rendered obsolete, nevertheless since this can no longer be redeemed it is considered obsolete. Bonds however are more of a branch of collecting known as scripophily rather than numismatics.