Three months after an Angola-based invasion of Zaire threatened to touch off a proxy war of East and West, those two African nations are patching up their problems peacefully and the United States has decided to display its approval by releasing $26 million in previously withheld aid to Zaire.

State Department officials said the decision, made in recent days, will release $18 million in food aid and $8 million in security supporting assistance to Zaire. They said this was done after President Mobutu Sese Seko made "substantial progress and good faith efforts" to meet three U.S. and Western European conditions: negotiate a border pact with Angola, improve the status of human rights, and take steps to curb corruption.

No tangible gestures of approval for Angola have been made in recognition of that country's willingness to improve relations with Zaire and to discourage any repetition of the cross-border raids that alarmed Washington earlier this year. But Angola has been told of American approval and support for its recent actions, officials said, and they did not rule out some material U.S. steps if the Zaire rapproachment continues.

The moves by the two African states to defuse their conflict are a far cry from the East-West invective and threats of expanding outside intervention which dominated the world press in the wake of the attack into Zaire's Shab Province three months ago this week. In contrast with that public alarm in Washington and elsewhere, the strides toward harmonious relations have received only modest attention.

There is no consensus among Washington officials about all the reasons for the favorable turn of events or how permanent it is likely to be.

A White House policymaker said it is in Soviet and Cuban interest to cool off the recurrent insurgencies from Zaire targeted against Angola. Angolan President Agostinho Neto has enjoyed Soviet and Cuban support and has in recent weeks reached a working relationship with the United States. State Department officials say the new initiative with Zaire appears to be primarily due to Neto's decision rather than Cuban or Soviet influence.

There is general agreement that U.S. and European pressure on Mobulu was an important factor in Zaire's decision to improve relations with its neighbor. This pressure arose from Washington's conclusion that a political solution would be more effective in the short run than military action in helping restore security to mineral-rich Shaba Province.

To implement this idea, Washington took a surprisingly tough stand in the Paris and Brussels meetings of concerned Western nations in June, insisting on improved relations wtih Angola as one of the three conditions of any U.S. emergency aid to Zaire. President Carter backed up this stand with a personal letter to Mobutu, and other Western leaders did the same.

"it's not so much that this was planned as it is a mark of how swiftly things change in Africa," said a congressional observer of the recent Angolan-Zaire maneuverings.

The first important move toward a political settlement of the border problems was a little-noticed declaration by Angola's Neto on June 3 that rebels returning from raids into Zaire would be disarmed. U.S. officials were skeptical at the time, but they now say there is evidence that this is being done.

There are also clear indications, according to the officials, that Neto is making good on a subsequent promise to begin moving an estimated 200,000 Zairian refugees into new camps in the Angolan interior, away from the border which they have crossed in the past on recent raids into their homeland.

Cuba, which was accused by the Carter administration of equipping and training the Angolan-based insurgents for the May 12-13 cross-border raid into Zaire, has had little or nothing to say about the Neto government's recent actions in disarming and moving the refugees. About the closest it has come was a Havana radio dispatch last week reporting that Neto has pledged to provide the refugees from Zaire with "protection, worthy work and subsistence opportunities."

Zaire, for its part, is expected to try to seal its border against insurgent groups which have operated from Zairian territory to attack the ruling regime in Angola. State Department officials said there appears to be no truth in press reports that Mobutu has arrested Jonas Savimbi, leader of the Union for the Total Independence of Angola (UNITA). the most important of teh insurgent groups. But the officials cite Mobutu's willingness to agree to a border verification group form other African states as a sign of his sincerity in seeking to curb Zaire-based border raids.

The rapproachment between Angola and Zaire was worked out in a series of meetings starting late last month in several neutral capitals of Africa and in visits by Angolan and Zairian emissaries to each other's capitals. The United States played a small role, according to officials here, in providing a communications link between the two formerly hostile neighbors in their earliest meetings.