The United States ratified the Sixteenth Amendment in 1913, authorizing a federal income tax.
The amendment states that Congress may impose taxes on incomes without apportioning them among the states according to population. That wording addressed constitutional problems surrounding an earlier federal income tax, which the Supreme Court had struck down in 1895.
Congress passed the income-tax legislation that followed the amendment in 1913. The new federal income tax initially affected a relatively small share of Americans and had rates far below those associated with later wartime tax systems.
The amendment did not create every income tax in American history; Congress had enacted temporary income taxes before it. Its importance was that it clearly established a constitutional basis for an unapportioned federal income tax. The amendment is therefore often associated with the permanent modern US federal income-tax system.