Discussed here.

This is a classic example of the government's "camels-nose-under-the-tent" technique. The tax was sold to the suckers with the idea that this was how it would be, and only the very, very rich would have to pay.

Here's a link to the tax form. If you want to figure out the 1913 income levels, exemptions, etc. adjusted for inflation, apply a multiplying factor of 21.

Pretty good deal, huh? But it was amazing how fast that camel forced its hairy ass into the tent. By 1918, the tax rate was already 4 percent on incomes up to $4000, and the top rate was 73 percent, and it just went up from there.

A history of tax rates here.