Today's modest decline in the Dow Jones Industrials (DJINDICES:^DJI) stems more from concerns about Europe than from the ongoing struggles the U.S. economy has faced in recent years. The Dow and other U.S. stock markets haven't fallen nearly so hard today as exchanges in Europe, which closed down between 1.5% and 3%.

But it's important to remember that even companies based in the U.S. do business around the world. That makes Europe important to many U.S. companies, especially those with extensive operations there. Let's take a quick look at some of the Dow components with significant business in Europe.

Procter & Gamble (NYSE:PG)
A worldwide consumer powerhouse, P&G does business in more than 180 countries. According to its June 30 annual report, the company got about 40% of its revenue from the U.S. and Canada last year, but Western Europe makes up its next-largest segment, weighing in at 19%. When you add in a portion of the 14% it reports for Central and Eastern Europe, the Middle East, and Africa, you can see that what happens in Europe makes a big difference to P&G's results.

Johnson & Johnson (NYSE:JNJ)
Along similar lines to Procter & Gamble, J&J has worldwide scope as a health-care-focused mini-conglomerate. In 2011, the company earned about 26% of its revenue from Europe. But that understates the importance of Europe to J&J's overall business plan. If you look back five years, you'll see that while U.S. sales have been stagnant, European sales have jumped by more than a third. Given J&J's need for growth, the company can ill afford to have a weak Europe undermine its strategy.

McDonald's (NYSE:MCD)
It's surprising to see that an iconic American fast-food giant actually does more business in Europe than in the U.S., but the numbers have worked out that way every year since 2003. Last year, McDonald's got 40% of its revenue from Europe, versus less than a third from the U.S. -- and despite the importance of fast-growing Asian markets, Europe has actually contributed more of a gain in raw-dollar revenue terms than McDonald's Asian segment.