The U.S. first introduced an income tax in 1861, envisaging it as temporary measure to help finance the Civil War. An 1861 law imposed a temporary 3% income tax on incomes over $800 that expired in 1872.
In 1894, the federal government levied a 2% tax on incomes over $4,000, but the tax was struck down by the Supreme Court the following year on the grounds that directly taxing Americans' income was unconstitutional.
To overcome this ruling, President William Howard Taft proposed the 16th Amendment to the Constitution to permit the federal government to collect income tax. The Amendment was ratified in 1913 and Congress enacted a federal income tax shortly after.
Today, half of all federal revenue comes from individual income taxes.