A year ago, in the wake of President Trump’s tax cut, euphoric investors pushed the Dow Jones industrial average past 25,000, a record. The Dow had just gained 25 percent in 2017, and the Nasdaq had leapt 28 percent. Volatility was so low that there wasn’t a single day in 2017 when the S&P 500 fluctuated more than 2 percent.

Not everyone was celebrating.

“If there are any certainties, one will be that this party will eventually come to an end,” James Stack, president of InvesTech Research, told me a year ago. “And when it ends, it will end badly, and with high volatility.”

Mr. Stack turned out to be right. He lowered his recommended asset allocation for United States stocks from 82 percent last January to 72 percent in September, when stocks hit new all-time highs. He urged investors to raise cash in October, and at the end of November he recommended an even more defensive posture — including putting money in a fund whose value would rise when stock prices dropped. That brought his recommended net exposure to stocks to just 55 percent, the lowest since the depths of the last bear market in early 2009.

Stocks plunged in December, posting their worst monthly loss since the financial crisis and the worst December since 1931 and the Great Depression.