Out of the fog of war around the Horn of Africa there now emerges a pattern of events. And out of the pattern of events there emerges one more example of the basic evolution of the Carter administration. For in the Horn as elsewhere, the administration has moved from an early push toward a bold, new policy back to a more prudent attitude.

The key to what has happened is Ethiopia. For 25 years after World War II, the United States backed the Christian emperor, Haile, Selassie, with arms, military training for his soldiers and support for his claims to rule over the Moslem population of Eritrea in the north and the Ogaden Desert in the south.

In 1974, the emperor was outsted by a military junta, Dergue. Internal struggles pushed the Dergue to the left, and relations between Addis Ababa and Washington turned sour. By the beginning of this year, the Ethiopians had closed American military bases, and this country had ended its military-assistance program. To fill the gap, the Ethiopians turned to the Soviet Union.

The Ethiopians needed Moscow for two reasons: The secessionist movement in Eritrea was drawing new sustenance from a group of conservative Arab states led by Saudi Arabia and including Egypt and the Sudan. Addis Ababa wanted Soviet military help for the fighting.

The secessionist movement in Ogaden was getting help from Somalia, which in it turns had an armed force supplied and trained by the Russians. The Ethiopians wanted Soveit military and diplomatic assistance to prevent a takeover of the Ogaden by the Somalis.

When a Soviet effort to mediate between Ethiopia and Somalia fell through, Washington was presented with a golden opportunity. By acceding to Somali requests for American military help, this country could simultaneously one-up the Russians, pay back the Ethiopians and ingratiate itself with the regimes of Saudi Arabia, Egypt and the Sudan.

Just how much the United States moved in that direction is now in dispute. But I have no doubt that Arnaud de Borchgrave of Newsweek is correct in asserting that Washington tilted hard toward the Somalis.

The evidence is a comment made by the President to Vice President Walter Mondale for relay to Secretary of State Cyrus Vance and National Security Adviser Zbigniew Brzesinzki on April 6. The President's words are public because on April 6, Time magazine happened to be doing a day with Carter. According to Time, the President said:

"I want you to tell Cy and Zbig that I want them to move in every possible way to get Somalia to be our friend."

That pressure finally bore fruit on July 28 in an American commitment to provide arms to Somalia "in principle." But by that time a change in conditions on the ground was giving all concerned second thoughts. The Eritrean guerrillas and the insurgents backed by Somalia in the Ogaden had gone on the offensive in ways that threatened the dismemberment of Ethiopia. Dismemberment of Ethiopia raised the prospect of an unraveling of old colonial borders all across Africa, most immediately between Somalia and Kenya. As to the Russians, they were hung up in the worst way between support for their old friends in Somalia and their new friends in Ethiopia.

In these circumstances, it became clear that Russia would be seriously embarrassed even if - and maybe especially because - the United States did not play big two-power politics in the Horn of Africa. It was equally apparent that the Somalis wanted American support for more than merely defensive purposes. It was a question whether Washington wanted to promote the dismemberment of Ethiopia and the potential for border troubles all across Africa.

So in August Washington decided to hold aid to Somalia in abeyance. More recently new contacts with the Ethiopians were opened by a member of Brzezinski's national security staff.

Now matters are about where they stood when the Carter administration took office. The Ethiopians are on the defensive against secession and the Somalis, backed by the Arab states, are pushing forward. Russia has the headache of dealing with the area, and Washington is taking toward the Horn and attitude of reserve.

Thus in the Horn of Africa - as in relations with Russia and the Middle East, nuclear nonproliferation, withdrawal from Korea, and human rights - the Carter administration initiated a bold new departure, and then gradually returned to past practice. Well-founded criticism can be launched against false starts and exaggerated hopes. But on balance it makes more sense to be thankful that, because there were no crises, so little has been lost as the new administration finds its way.