A century ago, on Feb. 3, 1913, the 16th Amendment to the Constitution authorizing a federal income tax was ratified. But the amendment's adoption was more an accident than an act of political will, and tinkering with the Constitution was not even required for the federal government to tax Americans' earnings.

The country's first income tax was implemented to raise money during the Civil War. The tax was repealed in 1872 because the revenues were no longer needed. The idea was resurrected in 1894 as a populist measure to tax...