NEW YORK (CNN/Money) - For six weeks the stock market looked blithely on as the dollar weakened, rallying even as the greenback tumbled to ever lower levels against the world's other major currencies.

Over the weekend, everything changed -- Treasury Secretary John Snow's substantially retooled what the United States' "strong dollar" doctrine means, and suddenly Wall Street decided that the buckling buck was a problem. The notion that the dollar no longer has a safety net rekindled old fears that overseas investors will start pulling money out of U.S. assets -- causing more dollar weakness and snowballing into a general run for the exits.

That these fears have surfaced threatens to derail what Snow was trying to achieve. A weaker dollar appears to be part and parcel of a no-holds-barred effort by the Federal Reserve and Treasury to get the economy back onto its feet again. For the Fed, this means a pledge to keep interest rates low even if the economy begins to show signs of life, openly courting higher inflation as a means to fighting back the possibility of deflation.

For the Treasury, this means letting the dollar slide, which boosts the revenues of U.S. firms overseas and, because it raises the prices of imported goods, it also stirs the inflationary pot at home.

But for the Fed and Treasury to succeed, it's important that the marketplace has a high enough degree of confidence that the plan isn't going to go awry. Until Monday, this seemed to be the case. The dollar was dropping and Treasury prices were running higher, bringing yields down and lowering interest rates for companies and households. And the stock market was rallying, instilling confidence in investors and corporate types that things were looking up.

Monday's big drop in stocks raised the threat that the grand scheme could fall apart. If investors have decided the weaker dollar is a real problem, the scary run for the exits really could happen. That would be a financial catastrophe, seizing up the financial markets and threatening the United States with a fresh drop into recession.

The hope is that Monday's selloff was just a one-day event, not a portent of things to come. The S&P 500 had traveled over 17 percent from its March lows, after all, and it may be that Snow's comments were just an excuse to sell. But to be on the safe side, it's likely that Fed and Treasury officials will be spending a lot of time over the next week drumming up support for the great scheme. Let's hope they're convincing.  

-- Justin Lahart is a senior writer at CNN/Money covering markets and investing.