Why has the United States joined West Germany, France, Switzerland, and Japan in a combined effort to flood the market with dollars and thus cheapen their value?
''A mistake'' is how economist Herbert Stein describes the US Treasury's current effort to depress the value of the dollar by selling large amounts of the US currency in world money markets.
Intervention of this type, says Dr. Stein, a former adviser to Republican presidents and now a senior fellow of the American Enterprise Institute, ''gives the notion that we are going to intervene more than we intend to do.''
Treasury Secretary Donald T. Regan, charged with carrying out the intervention policy, in fact agrees with Stein.
Speculative movements ''between the dollar and the yen, and the dollar and the (West German) mark, are so huge,'' Mr. Regan has told reporters, ''that it would take tens of billions of dollars to counteract them.''
''The dollar is rising at a rate that is unbelievable,'' said a senior US monetary official in a telephone interview. ''We can't find any apparent reason for it, except a great deal of speculation.''
Speculation, the official said, stems from worldwide expectation that US interest rates are rising and will remain high so long as the United States runs huge budget deficits.
The purpose of the intervention, the official said, was to puncture this speculation by increasing the supply of dollars on the market and reducing the quantity of other currencies.
US intervention, coordinated by the Treasury and the Federal Reserve, took place July 29 and Aug. 1, according to a top US official. Central banks of West Germany, France, Switzerland, and Japan continue to intervene.
Other sources say that the Reagan administration may have tried, through limited intervention, to heed the pleas of allied governments, whose economies are hurt by high US interest rates.
As the dollar's value rises, commodities that are priced in dollars - above all, oil - become more expensive for other importing countries. This is true for Japan and the European powers now intervening to stem the dollar's climb.
High American interest rates also trigger an outflow of capital from other nations, because money managers and speculators seek high returns from dollar investments. This forces other governments to raise their own interest rates to limit the outflow.
Basic US interest rates, both short- and long-term, have climbed by more than one percentage point since late May. Mortgage rates now average 14 percent, and some experts expect the prime rate to rise above the present 10.5 percent.
''The major cause of the strong dollar,'' said a US economic official, ''is the budget deficit. While intervention may have a temporary effect, the dollar will remain strong until those deficits are perceived to be coming down.''
''So much money is floating around the world,'' he said, ''that the resources of the central banks cannot offset speculative pressures.''
Economist C. Fred Bergsten gives ''half cheers'' to the current intervention by central banks, on the grounds that they are ''trying to keep a bad situation from growing worse.''
Without intervention, said Dr. Bergsten, director of the Institute for International Economics, speculation is ''carrying us further and further away from underlying equilibrium'' between the dollar and other currencies.
Given the ''seeming impossibility'' of expecting US budget deficits to shrink , he said, the Treasury was correct to intervene along with other central banks.
Bergsten would like to see the Treasury intervene again, if and when the dollar begins to drop. Such intervention, he said, would be ''working with the trend, not against it,'' and would help to nudge the dollar lower.
Both President Reagan and Secretary Regan, however, are on record against official intervention, except in ''disorderly'' market conditions such as now prevail.