The first was the dollar devaluation of 1934 by President Roosevelt, when we defaulted on two-thirds of our national debt during the Great Depression by cutting the dollar's gold value by that amount. Currency devaluations are always very painful affairs, especially to ordinary people who are always the last to hear that their wages, pensions and life savings have been destroyed by the amount of the devaluation.
Devaluations are almost always unnecessary, though.
For a nation-state to repudiate its national debt by any amount is so thoroughly a confession of failure to its own people that even the poorest countries try to avoid it at any cost. When Lincoln floated the dollar in 1861, the intent was to restore its gold value after the war, for example. In 1902, one of the great economists of this century, Wesley Mitchell, demonstrated that even Lincoln's temporary repudiation of the government's contracts with its creditors was costly and unnecessary. In the world of classical, supply-side economics, devaluations are only imperative when you lose a war, as the creditors of the Confederate States of America discovered in 1865. There was, by the way, no discussion in Congress about bailing out the holders of Confederate bonds.