The American Civil War to be precise. The Revenue Act of 1861 was introduced to gather funds for the expensive conflict that lasted for four years, with the price of war coming in at approximately $2.5 million per day.

The first proposal to impose an income tax on America occurred during the War of 1812. After two years of war, the federal government had accumulated a then-staggering $100 million of debt. To fund the war against Britain, the government doubled the rates of its major source of revenue, customs duties on imports, which obstructed trade and ended up yielding less revenue than the previous lower rates.