FOR THE SECOND successive spring, it appears, Cuban-assisted Katangese soldiers have crossed from Angola into a traditional tribal area (Shaba) in southern Zaire, posing again the threat of grabbing Zaire's copper belt and setting up a separate state. The first sketchy reports suggest that this year the invaders are better prepared and that they are making a good deal more initial progress against the forces of President Mobuto Sese Seko.

It was widely noted last year that Angola, in sponsoring the Katangese, was "repaying" Zaire for Zairean support of the Angolan factions that the Neto government defeated in taking power in 1975. General awareness of that African background cost Zaire sympathy, if not support, for its travails. Yet Angola did not thereby acquire a right to sponsor annual invasions. It has no such right. It has, rather, an obligation to accept international standards of respect for the integrity of other states.

In its straitened circumstances last year, the Mobutu government cast about for foreign support, getting little from the United States. Much of American public opinion found Zaire and its appeal unworthy on various grounds, and the then-new Carter administration largely stepped aside, leaving it - in something of a cliffhanger - for Zaire's other foreign friends to fill the breech. Zaire is scarcely less straitened this year, and internal conditions have not substantially changed. But there is a greater awareness of the Cuban-Soviet presence in Africa. The attention that Mr. Carter himself now focuses upon it gives a country like Zaire, which seems to have fallen victim a second time to a communist-supported attack, that much more of a claim upon American aid.

Until the dimensions of the latest Shaba crisis are known, it is unnecessary to consider what the specifics of an American response might be. Kinshasa's first line of defense is, plainly, itself. Zaire's other foreign friends and the African community all have a stake in upholding the principle of territorial integrity and in putting a halt to aggression and tribal irredentism. Just as the administration seems more primed to counter another communist-backed advance in Africa, however, so the public would also probably be readier to go along.That is the difference a year's African experience has made.