Federal Income Tax

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I recently came across a documentary labeled "America - Freedom to Fascism" By Aaron Russo.
This video describes many of topics associated with the U.S. government yet primarily focuses on one thing, the ongoing subject of the Federal Income tax.
For years there have been many court cases and petitions asking for the IRS to provide the law making it mandatory to submit a 1040 tax form and pay the federal income tax. Each and every time the IRS and U.S. Government has been questioned about the legality of the income tax they have never commented as to whether it is legal or not under our own laws. Even some former IRS agents have gone on record saying that after several years of research they have not found sufficient evidence proving that we have to pay the income tax. The video also describes that the commonplace idea that money collected from this tax goes towards highways, education, and other such services is false.
If you haven't yet seen this video i strongly suggest you do. My own skepticism aside, i think it is a very interesting thing to think about.
So my question is, does the theory that the Federal Income Tax is illegal -or unconstitutional- hold any water, or is it yet another conspiracy theory?