BYLINE: Simon Kennedy and Matthew Campbell Paris

FOR the world economy, it's a case of US and them. That was the conclusion of investors, executives and policymakers as they ended their annual trip to Davos, Switzerland last week with the US poised to outpace world growth for the first time since 1999.

Celebrations were muted by questions over whether the strongest US expansion in 11 years will prove strong enough to power the global economy or whether it will be undone by slowing growth in Europe and China.

"The US can marginally help the world, but it cannot do it alone," former deputy US treasury secretary Stuart Eizenstat said. He said there was still a 50 percent chance that the drag from the rest of the world would prove greater than the momentum from the US rally.

The multi-speed global economy is already threatening the US by driving the dollar to an 11-year high against the euro, with few in Davos willing to bet against further gains for the greenback.

"Right now the US seems to be the greatest place in the world to invest," David Rubenstein, the co-founder of Carlyle Group, said. "The biggest problem in the US is that the dollar could become very... strong."

US economic might was on display throughout the four-day retreat in the Swiss Alps as executives such as Facebook's Sheryl Sandberg and PepsiCo's Indra Nooyi roamed the conference hall and companies from Salesforce to McKinsey threw splashy parties.

The largest economy is one of the few global hotspots as Europe seeks to avoid deflation, China slows and falling commodity prices hurt countries such as Brazil and Canada. …