For years, the U.S. warned Germany of building up a dangerous energy dependence on Russia, the source of more than half of its fossil fuel imports.

Europe's largest economy is facing up to the prospect that the bulk of its natural gas and coal supplies could get choked off, ripping through its industrial base and sparking economic upheaval. Companies including utility Uniper SE and chemical giant BASF SE are particularly exposed, and with gas reserves low, the pain would quickly spread to manufacturers and households already buckling under ever-rising bills.

"If either the Europeans no longer want to buy the gas or Russia cuts them off, that would be a very significant shock," David Folkerts-Landau, the chief economist at Deutsche Bank AG, said in a Bloomberg TV interview this week. "You will have a very serious recession."