16th Amendment
In 1895, in the Supreme Court case of Pollock v Farmer's Loan and Trust (157 U.S. 429), the Court disallowed a federal income tax. The tax was designed to be an indirect tax, which would mean that states need not contribute portions of a whole relative to its census figures. The Court, however, ruled that the income tax was a direct tax and subject to apportionment. This was the last in a series of conflicting court decisions dating back to the Civil War. Between 1895 and 1909, when the amendment was passed by Congress, the Court began to back down on its position, as it became clear not only to accountants but to everyone that the solvency of the nation was in jeopardy. In a series of cases, the definition of "direct tax" was modified, bent, twisted, and coaxed to allow more taxation efforts that approached an income tax.
Finally, with the ratification of the 16th Amendment, any doubt was removed. The text of the Amendment makes it clear that though the categories of direct and indirect taxation still exist, any determination that income tax is a direct tax will be irrelevant, because taxes on incomes are explicitly to be treated as indirect. The Congress passed the Amendment on July 12, 1909, and it was ratified on February 3, 1913 (1,302 days).