The unfunded liabilities with Medicare and Medicaid (of which the bulk of the liabilities come from, particularly Medicare) can be dealt with pretty easily, if the cost of healthcare is dealt with.
If we can enact policies that reverse the trajectory of health care costs, we could solve many of the issues facing the United States in the future.
Of course, we have to ask, how are we going to reverse the trajectory? Simply, we need to end crony capitalism in the health care industry. Obamacare is crony capitalism. Romneycare is crony capitalism. Medicare part D was crony capitalism. Nixon's HMOs were crony capitalism.
End the Government control and manipulation of healthcare, let the free markets work, and we can fix a large part of the future debts.
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There are so many other things that can be changed, too. $1tn of the US's debt can be attributed to the drug war. End the war on drugs. We'll see a USA with half the crime rate, half the number of prisoners, and people who are currently pushed off the sides of society becoming productive members of society.
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Then there's foreign policy. Get the fuck out of Afghanistan, Libya, Syria, Iran, Iraq, Pakistan. Close down 30-50% of all foreign military bases, and open a new one in the US for every 5 that is closed.
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Finally, the tax code. Tax credits and breaks need to be eliminated. Capital gains tax needs to be eliminated. Maybe merge welfare systems directly into the tax code, and introduce something like Friedman's negative income tax. (I don't like this, but it's a shit-tonne better than the current system).
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There you go, a handful of pretty much non-controversial policies that won't take away anyone's medicare, and will fix much of the deficit problem. Cut downs on the regulatory code, police spending, and eliminating some departments wouldn't go amiss, either (I'm looking at you, FCC).