Here's some links to good articles on how income tax came to be.
http://www.mises.org/freemarket_detail.asp?control=226&sortorder=articledate
http://www.freerepublic.com/forum/a182247.htm
excerpts:
Wartime "patriotism" made it possible to collect the 1862 tax, and Lincoln became the first president to tax incomes. Predictably, Congress increased the rates two years later. But the income tax had not achieved political legitimacy in peacetime. After the war, a falling deficit allowed the anti-tax forces to win the day. Rates fell, and, after much debate, the entire tax was scrapped in 1872--two years before it was statutorily set to expire.
The income tax began with class warfare - as an effort to soak the rich. And like that Russian proverb, the middle class ended up in the ditch it had dug for the wealthy. The tax passed from the rich not only to the middle class, but to nearly everyone.
To quiet the Democrats, Taft also proposed a Constitutional amendment to get around the Supreme Court. "The Congress," the amendment said, "shall have the power to lay and collect taxes on income, from whatever source derived, without apportionment among the several States, and without regard to any census or enumeration." But neither he nor other members of his party thought the amendment would ultimately pass.
-------------------------
Congress passed America's first peacetime income tax in 1894. It was progressive, not in its 2% rate, but in the fact that it exempted 98% of the population. It was simply class legislation against the rich.
The idea of taxing only the rich and letting everyone else ride freely on the government was good politics in a democracy, because the vote of the top 2% was insignificant.
Eventually, the Supreme Court weighed in on the 1894 income tax. The case, Pollock vs. Farmer Loan & Trust Co., was one of the most celebrated of the time. The court ruled 5-4 that the tax was unconstitutional. Because it was, in part, a direct tax, it had to be apportioned among the states by population, the court said