PARIS — Throughout Europe’s long debt crisis, Germany has prescribed the same strong remedy to its troubled neighbors: a stiff dose of fiscal discipline.

As long as the patients were southern European countries like Greece and Italy, seen as victims of an unhealthy lifestyle, northern-tier nations like France, Austria and the Netherlands have been willing to go along with Germany’s prescriptions for reducing debt in the name of economic health. And they were willing to support Germany’s insistence that the European Central Bank not be a lender of last resort to indebted governments by actively buying their bonds.

But suddenly, as investors’ fears mount that many euro area nations are about to tip into recession, even countries like creditworthy France are finding it much more expensive to borrow money in the open market. And with that development comes a dawning realization: that austerity, rather than making it easier for them to pay down their higher debts, could make it harder — and more expensive.

The exposure of the United States, and in particular its banks, to Europe’s debt problems caused a sharp sell-off in stocks Wednesday in the final two hours of trading. The Standard & Poor’s 500-stock index, flat on the day by about 2 p.m., lost 20.9 points, or 1.66 percent, to close at 1,236.91, after the rating agency Fitch warned that United States banks were vulnerable if Europe did not solve its crisis quickly.