The United States government did not initially try to collect the debt, as it was focused on rebuilding the country after the war. However, in 1871, the United States Congress passed the 14th Amendment to the Constitution, which stated that the Confederate government's debts were illegal and could not be repaid. This amendment effectively ended any chance that the Confederate debt would be paid.
In addition to the 14th Amendment, the United States government also passed the Ku Klux Klan Act of 1871, which outlawed the Ku Klux Klan and other similar groups that were formed to resist Reconstruction. These measures helped to suppress any attempts by the former Confederacy to resist the United States government's control.