On October 3, 1917, six months after the United States declared war on Germany and began its participation in the First World War, the U.S. Congress passes the War Revenue Act, increasing income taxes to unprecedented levels in order to raise more money for the war effort.
The 13th Amendment, which gave Congress the power to levy an income tax, became part of the Constitution in 1913; in October of that year, a new income tax law introduced a graduated tax system, with rates starting at 1 percent and rising to 7 percent for taxpayers with income above $500,000. Though less than 1 percent of the population paid income tax at the time, the amendment marked an important shift, as before most citizens had carried on their economic affairs without government knowledge. In an attempt to assuage fears of excessive government intervention into private financial affairs, Congress added a clause in 1916 requiring that all information from tax returns be kept confidential.
By then, however, preparation for and entry into World War I had greatly increased the government’s need for revenue. Congress responded to this need by passing an initial Revenue Act in 1916, raising the lowest tax rate from 1 percent to 2 percent; those with incomes above $1.5 million were taxed at 15 percent. The act also imposed new taxes on estates and excess business profits.
By 1917, largely due to the new income tax rate, the annual federal budget was almost equal to the total budget for all the years between 1791 and 1916. Still more was required, however, and in October 1917 Congress passed the War Revenue Act, lowering the number of exemptions and greatly increasing tax rates. Under the 1917 act, a taxpayer with an income of only $40,000 was subject to a 16 percent tax rate, while one who earned $1.5 million faced a rate of 67 percent. While only five percent of the U.S. population was required to pay taxes, U.S. tax revenue increased from $809 million in 1917 to a whopping $3.6 billion the following year. By the time World War I ended in 1918, income tax revenue had funded a full one-third of the cost of the war effort.