GUERRILLA LEADER Joshua Nkomo concedes that his men shot down the Rhodesian airliner in which 38 people died Sunday night. He denies that it was his men who then coolly murdered 10 of the 18 crash survivors - all civilians, some women and children. It was not the first atrocity of that war, which has seen repeated atrocities by regular and irregulars of both sides, but it was brutal and sickening, and its political effects may be severe. Coming just as many Rhodesian whites were looking for a way to compromise with the guerrillas - first of all with Joshua Nkomo - the incident may lead many of them to conclude they have no choice but to fight on.

Have the guerrillas of the Patriotic Front left them no alternative? Here is the way we see it: The black-white internal administration formed last May in Salisbury is crumbling. Its black members have not reconciled the guerrillas, as they promised to do, and its white members have not gotten up a visible momentum of real political change. The guerrillas are coming on so strong that Prime Minister Ian Smith was told by his high command early last month that the military situation was perilous. The economy is wasting. The sympathy generated for Salisbury by the latest guerrilla outrage can hardly alter the odds.

It was in these desperate circumstances that Ian Smith reached around his internal black partners to confer secretly in Zambia with Joshua Nkomo three weeks ago. Many white Rhodesians have long regarded Mr. Nkomo as the one guerrilla leader potentially moderate enough to make a solution whites could live with. Just what transpired in Lusaka is disputed. What is clear is that no breakthrough had been made by the time word of the meeting leaked last weekend. Did Mr. Smith risk his whole relationship with his black colleagues in Salisbury just to play games with Mr. Nkomo? Did Mr. Nkomo, speaking for guerrillas flushed with a sense of imminent victory, make Mr. Smith a fair offer?Outsiders cannot tell. But the hope must be that the two men have not exhausted their search for peace.

The United States continues to favor a conference attended by all parties; it doesn't think a settlement fashioned by any two of them would stick. When everything else has failed, American diplomats feel, all the parties may come to appreciate the diplomatic safety net the United States and Britain have rigged under them.

Perhaps so. But there is the risk of a self-fulfilling aspect to American policy: The United States, by withholding support or enthusiasm for a particular proposal, such as the internal settlement or a Smith-Nkomo deal, can weaken it and in that manner bring the parties to an all-parties conference. One must hope that by the time the parties get to such a conference, if they do, they will not be either too rigid or too broken to make something worthwhile of it.