WITH THE announcement of a slight reduction in the monthly trade deficit, the markets are reported to be temporarily placated. The markets -- that vast, restless collectivity of investors, traders and speculators -- are now the monitors of American economic policy, meting out swift and sure punishment for politicians' backsliding. How did the country ever get itself into a position in which policy has to answer to the speculators?

The trade deficit in September was $14.1 billion. At that level, the country needs roughly $14 billion of foreign investment to balance the deficit. Otherwise Americans would find themselves selling more dollars -- to pay for their imports -- than anyone wanted to buy, and the dollar's exchange rate would fall.

For the first six years of the Reagan administration, private investors abroad wanted to put their money, in huge quantities, into the dollar and into the United States. That balanced the trade deficits. But as time passed the foreign investors began to get anxious about having disproportionate amounts of their money in this one country, and they began to grow less optimistic about the future for profits here. Last winter the flow of foreign private investment in the United States abruptly dropped.

To prevent the dollar's exchange rate from collapsing -- and their own currencies from shooting sky-high -- governments in Japan and Europe had to step in and begin buying billions of dollars with their taxpayers' money. That happened last spring. The extraordinarily heavy new dependence on foreign governments' support is diminishing this country's freedom of action. This new reality underlies the recent assertiveness on the part of both the Japanese and the German governments in criticizing American procrastination and indecision on the budget deficits.

The implications have not yet sunk in here. Congress, for example, is writing a trade bill based on the assumption that Japan is absolutely dependent on access to the American market for its cars and other consumer goods and will have to conform to American wishes on trading practices. In fact, as Japan's government understands but most Americans do not (yet), the United States needs their capital as much as the Japanese need American customers. The Japanese statements on the budget deficit are a warning. Reduction of Japanese financial support is unlikely at the moment, but Japanese and European governments' support of the dollar indefinitely, at the present rate of $14 billion a year, is also pretty unlikely.

As some of the Europeans see it, the United States is demanding that they finance the deficits through the election year and enable the Reagan administration to get past the next 12 months without having to make any unpopular decisions. The quarrel between the U.S. Treasury and the German government over this demand broke into public in mid-October and was one of the incidents that led to the stock market crash.

The final stages of the trade bill may not be, as Congress supposes, negotiations with the White House to avoid a presidential veto, but rather negotiations with Japan and Europe to avoid a financial veto. As most people know, running up your debts leads to a loss of control over your own affairs. Six years of free spending and heavy borrowing under the Reagan administration have brought a historic shift of financial control from this country to its new creditors -- and financial control is political power.