HOW DOES THE international community deal with rogue regimes, those that under the color of national sovereignty commit unspeakable crimes against their own citizens? We have in mind not the mass deprivation of rights practiced by police states everywhere but the virtual genocide perpetrated by such regimes as Pol Pot's Cambodia and Idi Amin's Uganda. Pol Pot has already been driven from formal power. Idi Amin concerns us here.

President Amin seized power eight years ago, relying on his own small tribe and, in largely Christian Uganda, on the small Moslem minority. He murdered several hundred thousand Ugandans, most of them members of other tribes and Christians. Because of his reaching out to a Moslem constituency he won crucial aid from Arab countries. For religious and political reasons of their own, they cynically and myopically exploited his greed. The Saudis have given tens of millions of dollars. Egypt has helped. Palestinians welcomed Mr. Amin when he turned on Israel. Libya has at least twice provided arms and soldiers in times of military crisis. The common impression of Idi Amin as internationally isolated is misleading. He has heavy Arab support, plus comfort from the African rivals (Kenya) of his African rivals (Tanzania). And, of course, the Soviet Union has been behind him.

In the early 1970s, Tanzania's Julius Nyerere, who hates President Amin, loosed a force of Ugandan exiles on him-to no avail. Only last October did Mr. Amin, by invading and occupying northern Tanzania, give President Nyerere the cause he needed to overcome his reservations about intervention and to attach Tanzanian units to a second force of Ugandan exiles. President Nyerere is responding to an attack on his own territory. In undertaking to bring down Mr. Amin, he is performing a genuine service for Africa. He is serious enough to be willing to risk the severe embarassment of being beaten back by the well-equipped libyans, who seem now to be doing almost all of Idi Amin's fighting. The current battle for Kampala may tell.

So arbitrary are Africa's national boundaries and so fragile are its political systems that it makes almost everyone jittery to see troops crossing an established African frontier. Yet when so many countries have acted this badly in a given situation, it seems hypocritical to say border-crossing is never justifiable. Our hope now is for a quick victory by the Ugandans supported by Tanzania, for the departure of all foreign forces, and for the relative calm in which Ugandans can rebuild their broken and multilated nation.