Peace talks have started in Lebanon, presumably because the fighting is over, so it is more than a little noteworthy that Israeli and Lebanese soldiers have been so busy the past few days stringing barbed wire around the negotiating site.

At the moment it appears as if the Begin government has achieved its war aims in Lebanon. Israel invaded its neighbor to drive the forces of the PLO away from its frontiers and establish a Christian republic to govern the country.

One need not be cynical about the concept of a Christian republic. Though anachronistic, it may be the only way Lebanon can be ruled. The Middle East is not famous for pluralism--not Israel, not Syria or Egypt or Iraq, not Lebanon. The classic way for different peoples to be governed within a single jurisdiction in the Middle East is for one to dominate the others, or for all to be dominated by an outside force.

As long as the Christians ruled Lebanon, as they did for the three decades after the French left in World War II, there was peace. But as the Moslems rose to challenge them, the social cement became unstuck, and factions that had for centuries lived in unfriendly proximity began to kill. In 1975, when the civil war began, the killing took on massive proportions.

In invading Lebanon last summer, the Begin government reasoned that it could restore stability, and thereby secure its northern border, by reestablishing Christian dominance. When assassins deprived the Israelis of the enthronement of Bashir Gemayel, the tough leader of the Christian militias, they took the next available candidate, Bashir's brother, Amin.

But since the fighting stopped in August, the Israeli government, contrary to its pledges, has been supplying arms not to the Lebanese army, the heterogeneous force under the authority of Amin, but to the Lebanese Forces, which are the Christian militias once commanded by Bashir, and the perpetrators of the Sabra and Shatila massacres.

It is no secret that Begin does not trust Amin Gemayel, whose political ideas are far more complex than his brother's. Begin is negotiating now only because the American government insists on Israeli departure from Lebanon, and he will not go if he cannot obtain for Israel a reasonable return.

Within the framework of war, Begin's is not an untenable position. Except for Israel, Gemayel would not now be in office. Begin did not send the Israeli army into Lebanon to substitute one unfriendly regime for another.

Amin Gemayel has waived the support that his brother once received from the Begin government, but, as he sees it, the price would have been even greater if he consented to Lebanon's becoming an Israeli client state. Begin no doubt intended precisely that for Lebanon under Bashir, though he was by no means certain of Bashir's consent either.

Amin, in turning his back on Israel, has staked much on reaffirming Lebanon's Arab indentity, which will make him eligible to draw for aid and investment on Arab riches. But it may not be enough. The question he faces is whether his government can establish its credibility, and thus its authority, within a Lebanon that remains as murderously divided as ever.

Hardly a day passes without news reports from Lebanon of battles involving Christians and Druzes, Sunni and Shi'a Moslems, rival gangs of left and right, Palestinians, Alawites, what-have-you. The barbed wire being strung around Khalde, where the Israeli-Lebanese talks are being held, is precisely to protect the negotiators from such sectarian warfare.

Practically everyone agrees that the purpose of the negotiations is to get the foreign forces out of Lebanon--Israelis, Syrians, the remnant of the PLO. But no one has addressed seriously who will keep the order in this intrinsically chaotic society after the foreigners leave.

Amin has asked the United States, France and Italy, who already have 4,000 men in Lebanon, to send 5,000 more, and he is looking to other Western governments for still more troops. Perhaps the new foreign forces will help the Lebanese government to build a semblance of political stability.

But there will be much turmoil before that goal is achieved, and even if Israelis and Lebanese agree on peace terms, the shape of Lebanon a few years from now is likely to look far different from what Begin intended when he went to war in June.