HOW IS IT that a crazy like Idi Amin of Uganda gets to be able to invade Tanzania? It's that the Soviet Union, prowling for pawns to move on the African chessboard, loaded up President Amin with the military hardware he has now put to aggressive use against his neighbor. Without Soviet guns and planes - and the encouragement they provide - Ugandan forces would not be sitting on some 733 square miles of northern Tanzania. Amin's claim that he is merely responding to a Tanzanian invasion has no demonstrable basis at all.

The Russians also supply guns to Tanzania; there the Soviet purpose evidently is to displace Peking as Tanzania's leading military patron. This puts Moscow in the position of, in effect, sponsoring one client's aggression against another. Other African states have a powerful interest in persuading the Soviet Union to cut off Uganda. By doing so, Moscow would at once be aiding one of the most progressive African states, punishing the most savage regime in the continent, and bolstering the essential African principle of respect for colonially drawn frontiers. Otherwise, since geography and the deployment of forces make it hard for Tanzania to drive the Ugandans out, Tanzania's julius Nyerere could be in a very difficult straits.

One further possibility is that Kenya, through which all of land-locked Uganda's oil and other imports pass, could staunch the flow. The trouble here is that Kenya and Tanzania have their own nasty feud going. They now have fresh and urgent reason to talk out their differences. Idi Amin is an embarrassment and menance to all of Africa, and all Africans share an interest in seeing his occupation of northern Tanzania rolled back and Amin's own rule brought to a end. He is no less loathsome, we might add, for playing the buffoon: the other day, Amin, a hulking former boxer, challenged the slight Nyerere to settle the war in a boxing match.

The United States has sharply condemned the Ugandan invasion. Just last month, trade with Uganda was halted in response to Amin's gross human-rights violations, including his butchery of hundreds of thousands of his own citizens. As it happens, the cutoff left Washington with no easy way, beyond words, of talking concrete steps to help out Tanzania in the current crisis arising from actions Amin has taken outside his own borders. That means in practical terms that his invasion has to be dealt with either by the Russians or the Africans, in their respective ways.