ECONOMIC WARFARE HAS now broken out between the United States and Iran on a scale that the world has not seen since 1945. Freezing another nation's bank accounts is a portentous step, for it implies further actions by both sides -- and by other governments as they are drawn into this widening conflict. To keep the freeze effective will require at least the tacit cooperation of banking authorities abroad, particularly in the European financial centers. Because most large American banks now have branches abroad, freezing bank deposits is not quite so simple as it was a generation ago. A great deal now depends on Europeans' reactions, and whether they support the judgment that Iran is now an outlaw regime that, by taking diplomats hostage, has put itself beyond the pale.

The president's order to freeze the assets was a reaction to a new wire account that the Iranians were about to pull them out. The Treasury justifies it as a precautionary move to protect future American claims for damage or confiscation. That's hardly the full reason for this drastic action.

The Iranians apparently intended to move their money -- several billion dollars -- from New York to London or perhaps Frankfurt. If they had done so, the New York banks would immediately have borrowed it back, at a cost of a hairline difference in interest rates. While that would have affected the banks' profits, it lwould have had little effect on the world's monetary system -- as long as the money remained in dollars. But if the Iranians had begun to trade it in and out of other currencies, in a deliberate attack on the stability of the dollar, that might have had a more serious impact. Since the Europeans are not anxious to see their currencies driven up by the tactics of a volatile and unpredictable revolutionary regime, they have a certain self-interest in not welcoming the Iranian money into their banking systems.

But Iran continues to sell oil -- probably about $75 million worth a day, with the price rising very fast. Their earnings in two or three months would be the equivalent of the highest estimates of the asset fro- zen in New York. The Iranians' next move may well be to demand payment in other currencies.

What countermoves does economic warfare next offer to the United States? They are not of a nature that ought to be taken without long and careful thought. There has been talk of an embargo on American food exports to Iran. That is an ugly weapon, from which the least culpable are the first to suffer, and it is rarely effective. There are too many sellers, and resellers, of grain.

The next move lies with the regime in Tehran. It is the Iranians who seized the hostages, and the Iranians alone who have the power to end this intolerable situation without further harm to those hostages. But as Iran's isolation deepens, the revolutionary government may well try to use the oil itself as a further hostage against the world. Perhaps other nations now need to brace themselves to do without Iran's oil altogether.