Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

The Dow Jones Industrials (^DJI 0.56%) are rebounding somewhat from their recent losses, with the average up about 90 points as of 12:15 p.m. EST. But even with today's gains, the Dow is down more than 1,100 points since the beginning of the year, and the sudden reversal in the U.S. stock market has sent ripples around the world.

The hardest-hit major market internationally has been Japan, where the Nikkei 225 (NIKKEIINDICES: ^NI225) is already well into official correction territory, down 14% since the beginning of the year. Despite aggressive economic reform promises from Prime Minister Shinzo Abe, the growth that the Japanese economy has seen has looked fairly fragile, especially in light of concerns from neighboring China and the anticipated slowdown in manufacturing that many see as likely from the emerging-market giant. A rise in the Japanese yen also threatens to reverse much of the progress Japan made in 2013, when its stock market soared 57% in local-currency terms and lifted the dollar-denominated iShares MSCI Japan ETF (EWJ -0.60%) to a 26% gain.

In Europe, declines have been much less extreme. Stock markets in France, Germany, and the U.K. are all down between 4% and 4.5%, but more broadly, many investors see value in European stocks as the eurozone economy begins to emerge from recession. Especially after last year's big run-ups in the U.S. and Japan, bullish investors hope Europe will follow the same course that U.S. stocks did after the 2008 recession, with extensive share-price gains lasting well into the recovery period.

Clearly, emerging-market stocks are most vulnerable to the global economic issues of the day, as an extended slowdown in growth rates and the threat of contagion from some of the weaker emerging economies combine to create fear among even the biggest markets in the emerging world. The iShares China Large-Cap ETF (FXI -0.29%) and the iShares MSCI Brazil ETF (EWZ 2.09%) are both off about 12% for the year, pulling down broader-based emerging-market ETFs in sympathy. As inflationary pressures build in Brazil, India, and other key rising economies, what happens in the emerging world could well set the course for U.S. markets in 2014.

Even when the Dow is struggling, it's important to keep watching international markets for signs of particular strength or weakness. By pinpointing weak links in the global economy, you can anticipate which individual companies will benefit the most and which are most prone to possible declines.