On this date in 1862, Congress formed the Bureau of Internal Revenue to pay for the expenses of the Civil War.
From Wikipedia and Reference.com:
In July of 1862, during the Civil War, President Lincoln and Congress created the office of Commissioner of Internal Revenue and enacted an income tax to pay war expenses (see Revenue Act of 1862). The position of Commissioner exists today as the head of the Internal Revenue Service.
This organization was created to enforce these taxes named for the internal revenue to be collected (and was formerly called the "Bureau of Internal Revenue"), in contrast to U.S. government institutions that collected external revenue through duties and tariffs.