Shareholders of General Motors (NYSE: GM) have had a rough week. The stock dropped 10% on Wednesday, after executives scaled back fourth-quarter expectations on concerns about economic conditions in Europe and (to a lesser extent) South America.

It's up a little bit since then, but it's unlikely to bounce in a big way anytime soon because the concerns are real. GM's pretty healthy, all things considered, but challenges around the world mean that growth might be scarce for a while.

But a big drop like Wednesday's raises the question: Is this drop an opportunity to buy? Or is it time to steer clear of the General?

Big challenges may limit growth for a while
GM has a lot going for it -- improving products, improved margins, and a strong management team that has moved swiftly to make the changes GM has sorely needed for decades. A few years from now, GM's dramatic turnaround may well look a lot like Ford's, with GM remade into a global powerhouse. That profitability and solid execution have made it an appealing-looking investment, even as its share price has languished in recent months.

But here's the thing that has investors worried now: Things that are (mostly) outside of GM's control may limit growth for a while. Take Europe: CEO Dan Akerson and CFO Dan Ammann had hoped that GM's long-troubled European division would break even in 2011. Now they say it won't happen, and the region lost $300 million in the third quarter. Part of the problem is macroeconomic, but GM Europe should be able to make money in all but the direst economic conditions, Akerson says, just as its restructured North American branch now can.

Changes to make that happen are already under way -- to his credit, Akerson doesn't waste time when faced with a problem (another big change from the old days). Already, GM Europe's longtime president is being replaced, and responsibility for marketing the Chevy brand in Europe has been reassigned. It's a safe bet that more changes are in the works.

But Europe's not the only problem. South America, where the product portfolio is dated relative to competitors and -- again -- costs are higher than they should be, is another place where hoped-for growth isn't happening. The region had been profitable, but just broke even last quarter. Again, fixes are already in the works, with a slew of new products set for launch in the coming months and layoffs and other cost-control measures under way. But while the problems aren't as big as Europe's, South America is another region that needs to be brought in line with GM's goal of sustainable profitability even in tough economic times.

Even China, where GM sells more vehicles than it does here, isn't delivering the big boost to the bottom line that you'd expect. GM's operation in China made about $400 million last quarter, an amount that was largely offset by losses in GM's other overseas operations -- and China's growth is slowing, even as GM continues to invest heavily in product development and new factories there.

North America is GM's strongest region and is very unlikely to swing to a loss, but even those profits could be squeezed in coming months. As Toyota (NYSE: TM) and Honda (NYSE: HMC) return to full production after a year of natural disasters and seek to regain lost market share, competition for buyers will become fierce. GM's pricing may come under pressure, and the company may need to increase spending on incentives or risk losing sales. I do expect the company to stick to its recent pledge of keeping its incentive spending roughly in line with the industry average, but Toyota is widely expected to spend heavily, so that average may rise.

The case for GM is still solid, but only if you're patient
Investors (and potential investors) shouldn't lose sight of the fact that the strength of GM at home is a radical change. It means that not only is GM's survival no longer remotely in question, it's likely to remain profitable unless the economy hits massive new lows -- and even if the worst happens, the company has more than $30 billion in the bank that it can draw on to keep its efforts on track. That strength means that GM's current leaders can make sweeping long-term plans for an enduring global turnaround, without worrying about whether the company can get through the next year or two.

Akerson's tenure may have started with some stumbles, but he and his team have in fact made long-term strategic plans that are both audacious and impressive, setting in motion a massive effort to consolidate the General's global product portfolio among a smaller number of models and platforms. It's similar to (and clearly modeled on) Ford's (NYSE: F) "One Ford" plan, which has resulted in a portfolio of strong, competitive products like the new Explorer and Focus -- and sustainable profits for the Blue Oval.

That's the real investing case for GM -- this is an enormous corporate turnaround that's still in progress, its chances of success look very high, and in a few years current prices are likely to look like a steal. But economic challenges around the world, and the fact that parts of GM still need work in order to meet those challenges, means that the stock price may not be going anywhere for a while. In fact, if the economy heads south, it may drop further.

I own GM and I'm planning to hold on to it. But whether you should do the same, or sell it to buy something more likely to give you growth in the near term, is a question you'll need to answer for yourself.

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