The possibility of a deflationary spiral and depression still seems to dominate  the Federal Reserve's list of worries. There has been some improvement in several economic indicators in the last couple of months or so, but this economic recovery from 2008 recession  still remains much weaker than any previous post World War II recovery . Real income growth for the average worker has been near 0 over the last 10 years.  The portion of long term unemployed as a percentage of all unemployed is double the normal ratio. The number of discouraged workers has increased, sharply lowering the labor market participation rate to 63.6 %. The Gold market mini-crash, seen by many as a bear raid by investment banks or even central banks, is on the face of it a deflationary event.
    By the standards of the terminology used prior to the Great Depression of the 1930's,  conditions that have prevailed for the last 5 1/2 years might very well have been labeled a depression. For that reason, I expect the Fed to continue with the QE and zero yield short term rates. As long as earnings can slowly increase the equation PV = FV/ (1 + r)^n.  will keep the stock market rising. PV= Present Value, FV = Future Value (future cash flow), r = annual interest rate, n= number of years of compounding. The ^ symbol is used on non-mathematical keyboards to indicate an exponent. "x^2" means "x squared". This formula is the reverse of the compound interest  formula and is thus derived from it.
   If interest rates are low, the value of r is less,  FV is being divided by a smaller number, and thus the formula's quotient yields a higher value for PV, which is the discounted Present Value of stocks (what they are selling for now). This is closely related to the idea that in a zero interest rate environment, there is no sacrifice of interest required to invest in stocks (there is no low risk, interest yielding alternative to stocks). If earnings decline, the FV part of the formula will decrease and thus the quotient  PV will diminish for that reason. If interest rates increase, the formula quotient PV will fall because FV is being divided by a larger number . That is why I say that a slow growth economy with gradually rising earnings and very low interest rates will cause the bull market to resume after the correction completes. If any of these factors changes significantly, the correction could easily develop into a bear market.
   I wrote in Geopolitics of  a Reserve Currency on April 1, 2013 that I thought that the countries under the umbrella of protection of the US military "understood" that a large part of the US deficit was caused by military spending and for this reason would not try to topple the Dollar as a Reserve currency. In other words part of the US deficit increase was incurred on their behalf so that it would not only be ungrateful, but foolish for them to undermine the Dollar.
    The agreement between the Saudi government and the Nixon administration in 1973 which stipulated the the US would indefinitely guarantee the protection of Saudi Arabia (and probably the other Persian Gulf States) in exchange for the Dollar being defined as the exclusive means of payment for Arabian oil.But this agreement requires that the US continue to maintain a powerful, state-of-the-art military. If the US engages in too much deficit spending for domestic purposes, and reduces military spending to pay for it, that behavior would constitute reneging on it's half of the agreement.
   The purchase of US Treasury notes and bonds by our allies is a vital part of this agreement. In lieu of compensating us directly for the protection we provide, they accept the Dollar as a reserve currency and purchase our Treasury instruments (debts) thereby financing defense (and domestic) spending. The benefit of receiving reserve currency status is so great that it is apparently considered superfluous to pay the US directly. The one exception that I know of was allied (especially Japanese) contributions to the cost of the first Gulf War, Operation Desert Storm.
      The economy is supposedly strengthening enough to cause the US Federal Reserve to reduce it's US Treasury and Fannie Mae and Freddie Mac bond purchases (QE 3). If this happens, how much will it upset the discounted present value equation described above? It will probably cause longer term rates to rise a little bit, but if the economy is growing stronger, earnings should increase a little faster. The net result under this scenario is that the divisor in the formula will increase a moderately, but the earnings being divided into will do likewise. The result is that discounted present values of corporations will continue to rise. Since the stock market has been rising faster than present values this year, it may well need to give up some it's recent gains for stock price valuations to return to a better alignment with fundamental valuations (PE ratios, book value, etc).
    But it is rather evident that the Fed will increase the pace of QE 3 again if the recent economic growth proves to be a flash in the pan. So the only force I see that could trigger a bear market is simply that investors will have grown too exuberant and pushed the market too far above reasonable valuation. In other words, as Richard Russell of Dow Theory Letters puts it, the buying will finally exhaust itself. The usual caveat, of course is that the world financial system does not tear itself apart at the seams before investors get a chance to exhaust themselves.