In 1909, the 16th Amendment proposed to be added to the US Constitution:
“The Congress shall have power to lay and collect taxes on income, from whatever source derived, without apportionment among the several States, and without regard to any census or enumeration.”
It passed Congress in 1913 with the required two-thirds majority vote. It was signed by President William H. Taft and processed for ratification by the State governments. The amendment was ratified by 42 states at a time when there were 48 states in the Union. The income tax was nothing new. The use of income tax was passed as the Revenue Act of 1861 in order to fund the Civil War on income exceeding $800 that was subject to a 3% flat tax.