Moody's Just Downgraded the United States' Pristine Credit Rating -- Here's What History Says Happens Next for Stocks

There's never a dull moment on Wall Street. Earlier this year, the bulls could do no wrong, with the benchmark S&P 500 (^GSPC -0.67%) climbing to an all-time record high. Not long thereafter, a historic bout of volatility arrived. In April, the steady-footed S&P 500 endured its fifth-worst two-day…