TWO IMPULSES arise from numbed contemplation of the latest stage of the Lebanese horror show. The first is ceaseless wonder at the capacity of the Lebanese not only to inflict but also to endure death and pain. Tens of thousands of civilian casualties have been suffered over a period of 10 years of civil war, foreign intervention and terrorism, but the passion with which the struggle is pursued seems not to have abated. Anger at foreign intrusions outlasts foreign withdrawals and boils on, directed at fellow Lebanese.

Fantastic explosions of the sort that quickly drove American forces out of Lebanon happen again and again in Beirut and are treated as routine. The other day, for instance, a car bomb killed some 80 people, including a group of children who chanced by. Who did it and why are unknown. The Lebanese reel under the impact; some of them, of course, vow revenge. Americans feel compassion but, fatigued, see no easy way to bring it effectively to bear.

Along with the wonder, there is among Americans a pervasive confusion about what is going on. The latest battle in Beirut illustrates the difficulties. PLO forces evidently were trying to reestablish a presence of sorts in three Palestinian refugee areas or camps -- Sabra, Shatila, Bourj el-Barajneh -- in the city's heavily Shiite suburbs. Amal, the Shiite militia, fearing that re-creation of a little Palestinian "state within a state" would draw Israeli reinvolvement, went in after the PLO.

The Syrians would surely like to trim all of Lebanon's militias down to size and assert their own hegemony, but in this instance, being close to Amal and also to some of the PLO units, they seem mostly to be letting the fire burn out. Not content to fight house to house, both sides have been firing artillery -- this in a city. The Shiites, who were outraged when Christian militiamen slaughtered Palestinians in Sabra and Shatila in 1982, are reported to have sent squads into hospitals to kill Palestinian patients.

Lebanon is a graveyard: for its citizens and their hopes of comity and for the plans of others to weave the political design of their choice. Whether the Syrians, the residual interventionists in Lebanon, have the touch -- they certainly have the toughness -- to make their design stick is the key question. The most but also the least that Americans can do is to hope that the Syrians end the killing, and meanwhile to mourn.