Is the party over for Detroit's Big Three?

The No. 2 automaker scaled back its profit expectations for North America next year, citing a robust series of new-car launches as the main culprit holding operating margins to around 8 percent to 9 percent of sales. That's a slip from as high as 12 percent in recent years.

Ford said its global pretax profit next year could drop to as low as $7 billion, a significant decline from the expected $8.5 billion for this year (which, by the way, would make 2013 one of the most profitable years in company history).

Investors reacted badly, sending the stock down more than 6 percent by the day's close to $15.65. That's nearly an 8 percent drop from early December.

And just like that, Ford began to lose its standing as the investor darling of the auto sector. The company has won high praise for its handling of the financial crisis, and it -- along with the rest of the industry -- has made huge gains in the last few years as auto sales soared. In the U.S., auto sales have climbed by more than 1 million every year since 2009.

So if Ford is seeing profit declines, what's happening to the rest of the industry? General Motors (GM) is in the midst of a broad overhaul, with a new CEO just announced, a long-awaited exit of the U.S. Treasury Department as a major investor and a surprising decision to pull Chevrolet out of Europe, squashing earlier ambitions to make Chevrolet a global brand. Chevrolet's marketing boss abruptly resigned this week.

Chrysler, for its part, has delayed an initial public offering until next year, with sources close to the deal telling The Wall Street Journal that tax issues prevented the issuance.

There are signs the industry is seeing rapid change. For years, the Big Three have seen easy sales fueled by the availability of credit and a growing consumer interest in more fuel-efficient cars. Japanese automakers were hit hard by the devastating 2011 earthquake and tsunami that sharply impacted production and inventory levels, and the Big Three swooped in to pick up market share.

Detroit's automakers will have to fight hard to protect their stakes in the sector, and that means staying competitive on prices and innovation while fighting new rivals in the form of Tesla Motors (TSLA) and European stalwarts such as Volkswagen and BMW, which haven't made the kind of inroads in the U.S. that they want.

There are numerous other challenges outside of North America. In Europe and South America, sales and production have suffered in the down economy. And the fight is just getting started in China, the world's largest auto market.

None of the Big Three are resting on their laurels. In fact, Ford's profit warning stems partly from the nearly two dozen vehicles the company will debut next year in an ambitious launch. GM and Chrysler are revving up the innovation as well.

So a one sense the party really is over for Detroit's Big Three. Sales growth is slowing from unsustainable levels, rivals are healthy and the competition is getting fierce. The easy gains have all been made. Now the hard work to keep all of those big gains begins.