SOMETHING HAS happened in the Caribbean basin that, had it gone the other way, would have brought half the Senate to its feet to decry the latest surge of the Red Tide. Guyana, a small struggling English-speaking country on South America's northern shore, has quietly decided to take what the experts have long regarded as the single most important step open to it to advance its development.

The step involves constructing, with World Bank assistance, a multibillion-dollar hydroelectric complex that will allow Guyana to cut its dependence on imported oil and convert its ample bauxite reserves into exportable aluminum. Before the bank would go along, however, it had to be assured that Guyana would pursue the policies capable of satisfying would-be international creditors. This is the significance of the new agreement Prime Minister Forbes Burnham is making with the International Monetary Fund, which, in return for substantial sacrifices on Guyana's part, is offering credits and, in the process, certifying Guyana for the big investments (public and private) that the Upper Mazaruni project will require.

Assuming that the project turns out reasonably well, its potential economic impact in Guyana is hard to exaggerate. But there is also an immediate political impact extending across the whole Caribbean. Mr. Burnham had gone far in recent years to identify his country with Cuba's economic and foreign policy designs. For his pains, however, he did not get the substantial Soviet and Cuban economic assistance he sought. On the contrary, he got next to nothing. He came to believe that Moscow was funding a damaging sugar strike conducted by his political opposition; later there were suggestions of Soviet weapons smuggling. Cuba, he found, cheated on a fisheries agreement. And so on.

Meanwhile, Mr. Burnham could see Jamaica, whose current government seems entranced by the Cuban way, sinking into an economic swamp and suffering alarming political polarization. All this is evidently what led him to try to ensure his country's future -- and, not to put too find a point on it, his future -- by turning clearly to the Western way.

Much could still go wrong: it always can. It seems safe to say, however, that the good guys have won one in a region where it was important to win one. Though the United States stands to benefit, it is not, strictly speaking, an American triumph. As only they could, the International Monetary Fund and the World Bank have provided the critical leverage and incentive. In fact, all those who want to see development forwarded in a democratic context now have an interest in seeing that Guyana makes it.