After weeks of big gains, The Dow (INDEX: ^DJI) closed down 0.13% today after falling 0.5% point earlier.

It's ordinary for traders to sell after a string of gains. But the last-minute negotiations between the European Union and Greece served as an added reminder today that the improving economic outlook that's been driving market returns could have vulnerabilities. If conditions in Europe continue to deteriorate (goes the worry), the slowdown in economic activity could spill over into its trading partners, including the U.S.

Greece is caught in a catch-22 -- the harsh austerity that the EU is demanding of Greece is almost certain to take a massive toll on its economy, making it even more difficult for the country to repay its loans. The only question left is whether something will get worked out that allows Greece to default in an orderly way that minimizes the damage to other economies and the global financial system. As recently as the fourth quarter, Bank of America (NYSE: BAC), JPMorgan Chase (NYSE: JPM), Citigroup (NYSE: C), and Morgan Stanley (NYSE: MS) all had significant exposure to stressed European markets.

Still, so long as Europe avoids a full-blown financial crisis, it could be more likely than most think that the U.S. economy escapes from the Europe's troubles more or less unscathed. As far as the financial sector goes, regional banks generally have far less exposure to European markets or trading results.

While the market's day-to-day fluctuations get all the headlines, it's important for us to remember that it's long-term performance -- not week-to-week price fluctuations -- that ultimately matter. If you're interested in one stock that our chief investment officer picked to crush the market in 2012, check out our brand new report, "The Motley Fool's Top Stock for 2012." It highlights a company that is revolutionizing commerce in Latin America. For a limited time, you can get instant access to the name of this company by clicking here -- it's free

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