A surtax, in its simplest form,'s essentially a tax levied upon a tax. United States The most recent example of a broadly-levied surtax in the United States was one imposed to help finance the Vietnam War during the Lyndon B. Johnson administration. It essentially consisted of calculating one's ordinary federal income tax liability and then adding another 10% to it -- the amount of the surtax. As the U.S. Income Tax Code at that time was an extreme example of progressive taxation, the surtax was… (More on Surtax)