The United States Constitution placed restrictions on the power of Congress to enact taxes.
"The Congress shall have power To lay and collect Taxes, Duties, Imposts and Excises [ . . . ] but all Duties, Imposts and Excises shall be uniform throughout the United States [ . . . ]Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers [ . . . . ] No Capitation, or other direct, Tax shall be laid, unless in proportion to the Census or Enumeration herein before directed to be taken."
Article I, § 8, Clause 1, grants to the Congress the power to impose taxes, but requires excise taxes to be geographically uniform. The Constitution states that all direct taxes are required to be apportioned among the states according to population. This basically refers to a tax on property as well as a capitation.
President Lincoln had Congress impose an income tax as a Civil War necessity, but the Supreme Court in Pollock v. Farmers' Loan & Trust Co. (1895), ruled that this had been unconstitutional, a decision which greatly limited the Congress' authority to levy an income tax. The 16th Amendment allows the Congress to levy an income tax without apportioning it among the States or basing it on Census results.
"The Congress shall have power to lay
and collect taxes on incomes,
from whatever source derived,
without apportionment among the several States,
and without regard to any census or enumeration."
On February 25, 1913, the Secretary of State Philander Knox proclaimed that the amendment had been ratified by the necessary three-fourths of the states, and thus had become part of the Constitution. An income tax, the Revenue Act of 1913, was shortly passed by the Congress. (courtesty:wikipedia.com)
I work in the Trade side of Customs and Border Protection, now part of Homeland Security. We used to be the U.S. Customs Service in the Department of the Treasury, and our old mission was "To collect and protect the Revenue." San Francisco's handsome eclectic renaissance-revival Custom House opened in 1911, two years before ratification of the 16th Amendment. Some people have said our building is a cross between a fortress, a palace and a bank. All three descriptions are apt. Before the income tax, Customs duties were the principal source of revenue for the federal government.
Customs duties paid off the debt from the War for Independence, financed the purchase of Louisiana, Florida, Gadsden and Alaska, besides paying for the Mexican American War. So over half the physical territory of the United States was financed with Customs duties. Today, although we still collect revenue, that is not our primary mission, which is tied to regulation of trade and anti-terrorism.