The history of income taxation in the United States began in the 19th century with the imposition of income taxes to fund war efforts. However, the constitutionality of income taxation was widely held in doubt until 1913 with the ratification of the 16th Amendment.
Article I, Section 8, Clause 1 of the United States Constitution assigns Congress the power to impose “Taxes, Duties, Imposts and Excises“, but Article I, Section 8 requires that “Duties Imposts and Excises shall be uniform throughout the United States“.
The first income tax was created in 1861 during the Civil War as a mechanism to finance the war effort. In addition, Congress passed the Internal Revenue Act in 1862 which created the Bureau of Internal Revenue, an eventual predecessor to the IRS.

The Bureau of Internal Revenue placed excise taxes on everything. In early 20th century the income tax enjoyed renewed support, and in February of 1913 the Sixteenth Amendment was ratified to the Constitution, thus granting Congress the power to collect taxes on personal income. The new system collected the income tax at the source, it is done today, where taxes are initially withheld before the income reaches the recipient. In 1914 The Bureau of Internal Revenue released the first income tax form.

The first year was a test run where people simply sent in their forms to have them checked by the bureau for accuracy without paying any taxes. By 1915 some people, including several members of Congress voiced concerns about the complexity of the income tax form, stating that it was difficult for some to prepare and file their returns.

The Revenue Act of 1916 began the practice of adjusting tax rates and income scales. The original income tax was 1% for the bottom bracket, which was comprised of income up to $20,000, and 7% for the top bracket which was comprised of income over $500,000. Revenue Act of 1916 raised the top bracket to $2,000,000 and raised the rate for the bottom bracket to 2% and the top bracket rate to 25%.