With September's Fed Funds rate cut of 50 basis points, stocks everywhere rocketed higher. The breadth of the market rise was as breathtaking as the size of the move. Everything moved. Everything.

Which leads to a question you might have asked yourself. This is the Federal Reserve of the United States, right? Its purpose in making the rate cut was to counter underlending among American banks -- to spur economic activity.

Among the thousands of foreign stocks trading on the American exchanges, many have minimal operations in the United States, and not much need to borrow here. So why did they rocket higher? It's easy to see why Toyota  (NYSE: TM) or Sony (NYSE: SNE) would spike -- they do huge business in the United States. But why did PetroChina add $14 billion in market cap on the tail of a rate cut in the United States -- in the week after the central bank of China raised rates?

There isn't a simple answer here.

Why you need to own foreign companies
In fact, at least part of the answer is "temporary investor insanity," brought on by the thought that the Fed was addressing the financial scare brought upon by the collapse in the subprime sector in the United States. Obviously, though, empty houses in Bakersfield have nothing to do with Chinese petroleum.

Yet the effect of the crisis has very much to do with how American investors can profit from Chinese petroleum, or German telephones, or Indian banks. Neither PetroChina, nor Deutsche Telekom (NYSE: DT), nor ICICI Bank (NYSE: IBN) has much direct exposure to the U.S., or to the U.S. dollar.

But if you're an American, or if your paycheck is in U.S. dollars, you do. And that's why you need to be more aware of international investing. Because the other reason that international stocks went crazy is because the market expects the U.S. dollar to continue its decline in value versus other currencies. Why does the market expect this? Well, that's what happens in a lower-interest-rate environment.

If you're paid in dollars, nearly all of your currency exposure is likely in U.S. dollars. This might not seem like a big deal if you live and work in Erie. But in a time when the Federal Reserve is stepping in to spur lending in the U.S. market, you're missing an opportunity to profit by investing overseas.

Lower rates = dropping currency
In the past five years, the dollar has done quite poorly against several currencies, including losing nearly 30% of its value against the euro. If you're investing in companies that denominate their earnings in those currencies (and have minimal U.S. dollar exposure), these are almost like free gains.

It doesn't need to be exotic. For example, U.S.-based investors in Agnico-Eagle Mines (NYSE: AEM) have done a lot better than their Canuck counterparts over the past year as the greenback swooned against the loonie. And guess what? With U.S. interest rates declining, chances are that the dollar will continue to remain weak.

The difference is dramatic: Over the past 12 months, U.S. shareholders saw Agnico-Eagle rise from US$40.16 to US$58.41. Canadian investors have seen the price go from CAD$47.50 to CAD$59.06.

U.S.-based investors were up 45% on the year. Their Canadian counterparts were up only 24%. The difference? The dollar's slide.

Now, I'm not saying that the dollar will continue to go down. But I am saying that investors who diversify away from their home currency do themselves a huge favor by looking overseas for investments. I'm also saying that a lower-interest-rate environment in the United States suggests that the trend isn't changing.

So, what now?
For the past year, I've run a service here at The Motley Fool called Global Gains. Each month, my team and I scour the world for great investment opportunities at fabulous prices. Since the newsletter began, we've absolutely thumped the return of the S&P 500. We've gone overseas to visit dynamic international firms like Brazil's GOL (NYSE: GOL), and we've kicked the tires of hundreds of companies plying their trades around the world. The great growth stories of tomorrow are as likely to be located in Costa Rica or Israel as they are in Silicon Valley, and we aim to find them. If you'd like to come on the journey with us, a 30-day free trial is yours with my compliments.

But really, even though I can't guarantee that the dollar will continue to go down against foreign currencies, the long slide of the greenback over the past five years has come in an environment of exceedingly low interest rates in the United States. If you think that condition will remain for some time, you have no excuse not to look across the pond for more of your investment ideas.

This article was originally published on Sept. 20, 2007. It has been updated.

Rhythmatic, systematic world control. Magnetic, genetic, dement your soul. Bill Mann is the lead advisor of the Motley Fool Global Gains international investing service. He holds none of the companies mentioned in this article. The Motley Fool has a disclosure policy.