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 16th 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.
