THE debt crisis in Europe has finally, and officially, washed up on American shores. Last week, the mighty Federal Reserve moved to help European banks that have been having trouble finding people who are willing to lend them money.

Some of these banks are growing desperate for dollars. Fearing the worst, investors are pulling back, refusing to roll over the banks’ commercial paper, those short-term i.o.u.’s that are the lifeblood of commerce. Others are refusing to renew certificates of deposit. European banks need this money, in dollars, to extend loans to American companies and to pay their own debts.

Worries over the banks’ exposure to shaky European government debt have unsettled markets over there — shares of big French banks have taken a beating — but it is unclear how much this mess will hurt the economy back here. American stock markets, at least, seem a bit blasé about it all: the Standard & Poor’s 500-stock index rose 5.3 percent last week.

But stock investors have a bad habit of dismissing problems in the credit markets until it is too late. Back in the summer of 2007, the stock market was roaring, despite obvious problems in the mortgage market.