THE OUTBREAK of fighting between Iran and Iraq demonstrates, once again, the extreme vulnerability of the world's oil supply line. It is exasperating to hear Americans say cheerily that the United States won't be much affected because little Iranian or Iraqi oil comes here. Iraq is now OPEC's second-largest producer, behind only Saudi Arabia, and the main Iraq oil port, Basrah, is in the area where the fighting is going on. Most of Iraq's oil normally goes to Western Europe and Jajpan. If that flow is disrupted, the Europeans and Japanese will naturally bid for a larger share of other supplies. In a truly dire shortage, the United States has agreed -- wisely and properly -- to share any available oil, domestic as well as foreign, with its allies.

Warfare anywhere in the Persian Gulf region immediately raises the spectral possibility of interference with shipping from other Gulf exporters, in particular Saudi Arabia. Nearly one-third of the world's total supply of oil moves by sea through the Gulf and the Strait of Hormuz at its mouth. Any interruption of that procession of tankers for more than a few days would create a convulsion of the world's econoy.

How well is the United States prepared for that kind of emergency? The country's first defense is supposed to be the Strategic Petroleum Reserve. Five years after Congress first established it, the reserve contains only 91 million barrels of oil -- the equivalent of 16 days' normal imports. For the past year, in deference to Saudi wishes, the Carter administration has not been adding to the reserve at all. Congress finally forced the administration to reverse itself, and within the past several days filling has started again. It's a bit late. Mr. Carter orginally said that he would have a half-million barrels in the reserve by the end of this year.

The second defense is rationing. You can discount that one. It would take about a year to put the rationing plan into effect. Preparatory work is under way, but it proceeds very slowly.

That leaves conservation. It can be enforced several ways -- by the gross inconvenience of gasoline lines, by higher prices that send more dollars abroad or by gasoline taxes that keep those dollars here to be rebated to Americans. Which do you prefer?

Conservation has made significant gains since the last oil crisis. For every seven gallons of gasoline and fuel oil that Americans burned two years ago, they are burning six today. For every four gallons of oil that Americans imported two years ago, they are importing three today. That's good progress -- but not good enough. The country is still terribly exposed to disruptions in foreign oil fields.

The combat reports from the Persian Gulf are a warning that there may not be time for a gradual, comfortable transition to lower oil imports. Congress needs to begin now to design an emergency gasoline tax that would rise sharply with the degree of sudden shortage. Congressmen who don't like voting for gasoline taxes might usefully start thinking, immediately, about the alternative.