THIS IS THE moment of truth for Yasser Arafat, chairman of the PLO. He is, or last night he still was, with his men, having just been photographed carrying a gun in Beirut. Such of his forces as had survived sat in a tightening ring held by the Israeli army and the Lebanese Christian militias. The Israelis were allowing speculation to mount on whether they would wait out Mr. Arafat's possible breakout, escape or surrender, or try to kill or capture him in order to complete the decimation of the "terrorist infrastructure" in Lebanon.

The isolation of the PLO is awesome. Every one of its Arab friends has abandoned it, politically as well as militarily. So indifferent are most of them, or so distracted by the larger twin threats of Iranian imperialism and Islamic fundamentalism that the usual litany of threats against Israel and the United States is simply not being recited. Syria, the patron best situated to help, last week put its own interests first. The Soviet Union waited until the PLO was all but gone before expressing concern--in terms offering no relief. Other states, though alarmed by Israel's thrust, have shrugged at the impact on an organization known after all for its contempt for Lebanon and for its terror.

The most telling sign of PLO isolation is that so few people anywhere have mourned the thousands of civilians, Palestinians and Lebanese killed and wounded in the Israeli attack. True, the PLO uses civilian cover. But Israel has fired indiscriminately, kept the United Nations and Red Cross from ministering to survivors and even ousted Palestinian doctors from hospitals as "PLO." The United States, though it has offered humanitarian aid, has done nothing one can detect to soften Israel's cruelties to civilians. Captured Palestinian soldiers, meanwhile, are falling under Israel's longtime policy of denying PLO soldiers the formal protections of the Geneva POW code.

As Mr. Arafat surveys the scene, then, he can see personal risk and the danger of a military rout, the seemingly permanent loss of the PLO's last physical sanctuary, an organizational future bound to be dominated by recrimination, self-inflicted violence and the imperatives of survival in the harshest conditions the Palestinians have known since 1948, and great personal humiliation and suffering. In sum, Mr. Arafat has led his movement and his people to disaster.

At last is he ready to contemplate the one step that could conceivably save Palestinians from further years of black tragedy--a statement that they are ready to live side by side with Israel in peace? It is a fantastic thing to expect of Yasser Arafat or any other Palestinian at this moment. It is the only sane thing.