We have had two warnings in the last two weeks of what may be coming in the aftermath of Israel's Lebanese invasion. In Rome, chief rabbi Elio Toaff spoke out against "a shameful explosion of anti-Semitism" uglier than any since Hitler's day. In Cairo, President Mubarak foresaw a ferocious new outbreak of international terrorism.

Rabbi Toaff was protesting an incident during Italy's general strike on June 25, reviving singularly painful memories. A quarter of a million workers were brought to the capital that day by their confederated trade unions--communists, socialists, Catholics--to demonstrate on economic issues. Marching past the synagogue on the way to their rally, groups of strikers paused to chant obscene slogans. "Ebrai al forno!"--"Jews to the Oven!" they shouted. One group, with evident forethought, brought a coffin along to place beneath the synagogue's plaque commemorating Jews who had died in the Resistance.

The story would be commonplace if some unregenerate rightists had done this. But these were militant leftists, marching with Europe's most radicalized workers, not one of whom tried to stop them.

Sympathy for the wretched Palestinian refugees trapped in Lebanon and rage at the Israeli invaders are hardly enough to explain this away. There is, in fact, a decade of grotesque political deformity behind it.

By now, the Palestine Resistance is so closely identified with the worldwide left that few still remember its original ties with the anti-Semitic right. Nevertheless, the right was there first. Europe's Black International, an assortment of ex- and neo-Nazis, fascists, Phalangists and Putschists, was a good 20 years ahead of an emerging Red International in embracing the anti-Zionist cause. The left joined in around 1968, when George Habash sent a team to hijack an El Al plane in Rome, marking the Palestinian guerrillas' fateful entry into Europe.

By 1969, the Black International was meeting in Barcelona to discuss ways and means of arming and training Palestinian forces in the Middle East, and of "collecting elements disposed to collaborate in acts of sabotage in Europe"; a delegate was there from Al Fatah. By that same year or soon after, left- wing terrorist rookies from West Germany, Italy, Sweden and Northern Ireland were going off to train in the Palestinians' Middle East camps. Not long afterward, a first joint hit team of Palestinians and left-wing Germans attacked a Jewish old-people's home in Munich, leaving seven dead.

From then on, Europe's Black and Red terrorist international worked side by side, using the same slogan against "imperialzionism," providing the same logistic and backup services, gun-running in the same or intersecting rings, dealing with the same Palestinian agents. The curious brotherhood lasted throughout the '70s. Evidence coming out recently in Italian courts suggests that it is with us yet.

Sooner or later, under these circumstances, some of the pathological Jew-baiting endemic to the ultra-right seemed bound to infect the ultra- left. We are seeing that now, under the stress of Lebanese events.

What we are almost certainly going to see as well, once the fighting in Lebanon is over, is a resurgence of international terrorism in grand style. President Mubarak has warned of that obliquely, pointing out that "a humiliating surrender" imposed on the PLO "could only lead to the spread of terror everywhere, which nobody in the world could want."

The proposition was put more bluntly some days earlier by Walid Jumblatt, Lebanon's intractable left- wing Druze leader and a close ally of the Palestinians' hard-core Rejection Front. In effect, he said, the PLO's Yasser Arafat was dead: if the Israelis didn't get him, his own followers would. No Palestinian chief who was thought to have favored a degree of moderation or diplomatic negotiation could survive the calamitous battle of Beirut, Jumblatt asserted. No grounds remain even geographically, let alone politically or militarily, permitting the Palestine Resistance to bargain from a position of strength. Thus, any further talk of bargain was out.

One sole course remained, he declared: "to mount a terrorist assault on every country in the West," starting but by no means ending with the United States, of a savagery unequaled so far.

Can the threat be made good? Yes, indeed. The machinery for exporting terrorism from the Middle East may have wound down somewhat since the mid-'70s when the notorious Carlos was running a team of 50 multinational, professional hit men, who were bombing, burning, hijacking and murdering their way across Europe, taking orders from George Habash in Beirut and his late military commander, Wadi Haddad, in Aden. But the skills required to rev up again are still available; the driving forces of bitter humiliation and frustration have never been so strong; and if Beirut may be lost, there is always Aden.

The capital of South Yemen, a military colony of Soviet Russia's by now, Aden has been the safest of privileged terrorist sanctuaries for years. Not being contiguous to Israel, it has little strategic use for the kind of standing guerrilla army that the PLO has just lost in Lebanon. But it is the one place in the Middle East where the remnants of that whipped guerrilla army can still be sure of a welcome, where Menachem Begin could never chase them with Israeli troops, where hard-lining Palestinian strategists have plotted their most spectacular terrorist strikes before, and would presumably be free to do so again. Judging from the mood abroad as the Lebanese drama grinds on, they would not lack for friends.