We're both kind of right. From: https://time.com/4289687/1942-tax-day-history/

"When the modern income tax was introduced in 1913, it was constructed as a tax merely on the very top earners and was essentially irrelevant to most Americans. That changed in 1942, shortly after Pearl Harbor. The entire nation was mobilizing for war, and money was desperately needed. At the time, the idea of running a deficit was seen as disastrous, says Joseph J. Thorndike, author of Their Fair Share: Taxing the Rich in the Age of FDR and director of the Tax History Project. To pay for the war, Congress passed a new Revenue Act that nearly doubled the number of Americans who would have to pay income taxes."