Before the Second World War there was only one major area of the world that was not self-sufficient in food -- Western Europe. One small amounts of grain were traded. Today, however, one out of every 10 bags of grain enters international trade. Seventy-five percent of this is provided by the United States.

Side by side with this development is the growth in the number of grain importers. The socialist countries of Eastern Europe, as they enter the affluent, meat-eating age, are becoming major importers.

The successful developing countries like Malaysia, Souh Korea and Mexico are developing a taste for wheat-and meat-based diets. And they have the money to pay for large amounts of imported grain to fatten their beef herds.

The most explisive increase of all comes from the OPEC countries. Food imports from the United States have risen from $500 million to $1.8 billion in fve years. Nigeria alone is expected by 1990 to be importing around 10 million tons of food -- almost as much as the Soviet Union imports now in an average year.

On top of this is the continuing and expanding demand for food imports by China and the poorer developing countries.

All the evidence suggests, then, that the demand for grain will increase, making the United States even more that it is already the linchipin in the world's grain machine.

This is a potenially ominous development. The United States grows most of its grain under rain-fed conditions. It is highly vulnerable to the vagaries of the weather. There are worrying indications that the North American Corn Belt may be on the threshold of a period of diminishing rainfall. More than that, it has come up against one of the limits of growth. The biologist Norman Borlaug, wo won the Nobel Peace Prize for his work on the "green revolution," says that North American agriculural research has realized 80 percent of the "genetic potential" of its present varieties of wheat.

This is why the most informed watchers of the grain scene are conviced that for the rest of this century grain prices are destined to move upward. And even if the grain is available, the poorest Third World countries with their 1 1/2 billion people will be edged out of the market by other counries that can afford to pay.

Is there only the prospect of mass starvation on the horizon? Nothing short of a revolution in the agricultural priorities in both the poor and the rich countries can avoid it.

For the short term, there must be a system of world food stocks. Discussions on these have been lingering on for five years only to stall finally in February. With grain prices so high and with the exporters doing so well, there has been no incentive for them to agree to a mechanism that will dampen profit levels.

In he end, however, if the Third World is to escape from the bondage of the Chicago grain dealers it must Knuckle down to the job of growing its own food. It must increase its use of fertilizer, high-yielding seeds, irrigation and multiple cropping. The richer countries need to help finance this with a degree of generosity so far found wanting.

Only if his is done can we be sure that in our lifetime we will be spared C.P. Snow's nightmare vision of "millions and millions dying. We shall watch them do so on our television sets."