The Confederacy and the Union had enacted national income taxes before, but none lasted. President William Howard Taft took office in 1909 and proposed an amendment to address the undertaxing of the rich and the overtaxing of the poor. The amendment passed right away but wasn't ratified until 1913.
At the time, the federal tax code was only about 400 pages long. This 2008 IRS report estimates the current federal tax code runs about 3.7 million words long, making April 15 more taxing year by year.