IT'S NICE TO KNOW that the Saudis don't intend to turn off the oil in order to force an Arab-Israeli settlement. We have the word of both Prince Fahd and President Carter for it, after their talks last week. But nobody in the United States really thought that the Saudis were likely to use the oil weapon, on their own initiative, to force the peace negotiations. The danger, now as always, is that fighting could start anywhere in the region, for any reason, and sweep the Saudis and their oil policies up in it against their expectations and their inclinations.

The oil weapon has been used four times in the Persian Gulf. The British government and the international oil industry invented it in 1951, when Iran nationalized the British concessions there. They boycotted. Iranian oil, and the drastic drop in oil revenues had as much to do with the subsequent overthrow of the Mossadeq government as the CIA's intervention did. In those days, wells in many countries - including the United States - were producing far below their capacity, and it was easy for the consuming countries to shift rapidly from one source to another.

After the Suez crisis and Arab-Israeli war of 1956, most of the Arab producers joined in an embargo against Britain and France. The Suez canal was shut, Syria sabotaged the pipeline from Saudi Arabia to the Mediterranean and the supertanker had not yet been developed. There was fuel rationing in Western Europe until the diplomats managed to get the oil flowing again. By the next Middle Eastern war, in 1967, the supertanker had arrived and gave the companies much greater flexibility. Iran's production shot upward. One crucial Arab state, Libya, still under a highly conservative king, refused to join the embargo. It collapsed after a month.

But when war broke out again in 1973, world oil consumption had risen enormously. The American fields were up to their limit. Iranian production could no longer be easily increased. Libya was under a revolutionary government that ardently joined the embargo.

Outside the Arab world there was no country able to increase production significantly, and the Arabs suddenly discovered that they had the power to put the price where they pleased. Even within the Arab world there was - and is - only one country with enough spare capacity to make much difference. That's why Prince Fahd of Saudi Arabia got such a heartfelt welcome here in Washington.

There are many possibilities for violence in the Persian Gulf region, by no means all of them involving Israel. Iran has large ambitions and is heavily armed. Iraq occasionally chooses to fan up the flickering revolution in Yemen. The ethnic rivalries around the gulf are as complex as they are intense.

The flow of oil from the Mideast has been interrupted once or twice in every decade since World War II. Yet the industrial countries, led by the United States, have let themselves slide into a position in which any interruption at all becomes intolerably disruptive and costly. President Carter and Prince Fahd offered the most generous of good intentions in their press conferences. But it will be the actual rate of U.S. oil imports, and not good intentions, that steers future events.