But there is one thing that World War I really moved forward—the income tax. The income tax required a constitutional amendment. In 1895, the Supreme Court had ruled such a tax unconstitutional, because it could put a disproportionate burden, on a per capita basis, on certain states versus other states. The Sixteenth Amendment is what made the income tax possible when it was ratified by the states in 1913. With the income tax, it soon became clear that you could use it to finance the American entry into World War I.
