Stocks around the world went nearly two years without a major bout of volatility. That long period of calm vanished within a matter of days.

While the Philadelphia Eagles were busy beating the New England Patriots in the Super Bowl, stocks in Asia slid sharply Monday morning, extending steep declines late last week in European and U.S. markets, including Friday’s 666-point drop for the Dow Jones Industrial Average. U.S. stock futures point to another lower open for Wall Street on Monday.

...