Many investors strive to put their money into shares of the best companies they can find. But if you're only looking within the U.S. for your stock picks, you could be missing out on the largest players in the global economy -- as well as the up-and-coming foreign companies that stand the best chance of becoming tomorrow's leaders.

It's not all about us
For decades, American investors were rewarded for staying close to home with their investment money. During the postwar boom, industrial giants General Electric and IBM diversified their businesses to take advantage of the most favorable economic conditions since before the stock market crash of 1929. Later, the rise of computers stoked the creative fires of Apple and Microsoft, turning early shareholders into millionaires. Of course, plenty of international companies also benefited from strong growth in the U.S. and elsewhere. But investors didn't have to diversify their portfolios geographically in order to reap strong returns.

Now, though, the tide is turning. Market mavens like Bill Gross and Jeremy Grantham are arguing that past expectations of investment returns from U.S. stocks are dangerously optimistic, and that the "new normal" of more modest rewards from investing lie ahead. But both of them, as well as many other influential investors, see the best opportunities coming from international stocks.

The new battlefields
What's causing the world's investors to turn their attention away from the U.S.? It's pretty simple: Over the past 100 years, the U.S. has grown from the world's leading emerging market in the early 20th century to a huge, mature developed market with an intricate web of companies doing business with and against each other. Innovation is far from dead here -- Intuitive Surgical's (Nasdaq: ISRG) robotic surgery technology and the video-streaming and DVD delivery company Netflix (Nasdaq: NFLX) are good examples of innovative companies that still mostly focus on U.S. customers.

But increasingly, investors are judging companies both within the U.S. and around the world on their exposure to the world's fastest-growing economies, such as China, Brazil, and India. Wal-Mart (NYSE: WMT) may have a huge advantage over U.S. rival Target (NYSE: TGT) right now, but its true competitors are international companies like Tesco and Carrefour, which do business on a similar scale around the world.

Where the winners are
That's actually good news for some U.S. companies. Yum! Brands (NYSE: YUM), for instance, doesn't have an inspiring domestic business, with revenue that has actually fallen in recent years. But its skill in penetrating the Chinese market has reawakened interest in the fast food giant. Similarly, Coca-Cola (NYSE: KO) and PepsiCo (NYSE: PEP) have a new place to wage their never-ending cola wars, with populations that are much larger than their domestic customer bases.

But in many cases, investors need to reassess which companies are actually the most promising in their respective industries in light of new global realities. Arguably, the U.S. is in the best geographical position to benefit from Brazilian growth, but Japanese and Australian companies are better-positioned to deliver the goods to China that it needs. Similarly, the big Wall Street banks that many think of as being the most important in the world will have to battle to remain relevant in a vast international market.

More importantly, as emerging markets continue to develop, you can expect the trend of industry leaders appearing outside the U.S. to continue. Successful businesses need to position themselves where the action is, and if the action keeps happening in exotic places around the world, businesses will follow suit.

Don't miss out
The key lesson that investors must learn is that if you want to own the best companies in the world, you can't have tunnel vision. Whereas owning only U.S. stocks worked reasonably well in the past, all signs are pointing toward the necessity of venturing beyond your own borders to make sure you really own stocks whose companies are truly on top of their industries.

Investing in international stocks is just one way you can improve your retirement prospects. Click here to read the Fool's new special report, The 7 Secrets to Salvage Your Retirement Today.

Fool contributor Dan Caplinger travels the world in his portfolio. He doesn't own shares of the companies mentioned in this article. Coca-Cola, Microsoft, and Wal-Mart are Motley Fool Inside Value picks. Intuitive Surgical is a Motley Fool Rule Breakers selection. Apple and Netflix are Motley Fool Stock Advisor recommendations. Coca-Cola and Wal-Mart are Motley Fool Global Gains picks. Coca-Cola and PepsiCo are Motley Fool Income Investor selections. Motley Fool Options has recommended diagonal call positions on Microsoft and PepsiCo. The Fool owns shares of Apple, Coca-Cola, ExxonMobil, IBM, Microsoft, Wal-Mart, and Yum! Brands. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Fool's disclosure policy works around the globe.