So often have the Israelis threatened to occupy southern Lebanon and eradicate the Palestinian guerrillas' capability to shell northern Israel that when the Israelis actually invaded there was little sense of surprise here.

For the question is: who would occupy the territory Israeli Prime Minister Menachem Begin says must be freed of Palestinian guerrilla artillery and rockets?

Taken at face value Begin's "Palestinian exclusion zone"--extending 25 miles from the Israeli-Lebanese border--would reach to within 15 miles of Beirut itself. The practical problem is who would enforce such directives and how.

Military specialists long have considered that it is one thing to clear the Palestinians out of their strongholds, but quite another to keep them out for good. The terrain is hilly and not easy to control without accepting a level of casualties that Israel traditionally has eschewed.

The Tyre pocket, that finger of Mediterranean coastline pointing down toward Israel could, according to analysts, be entrusted eventually to Shiite Moslems, once pro-Palestinian, but increasingly opposed to the lawlessness that has spread in southern Lebanon under the Palestinians. News Analysis News Analysis

Similarly, military specialists long have predicted that if the Israelis capture the fortress of Beaufort Castle, overlooking northern Israel, they may entrust it to their surrogates in the border strip commanded by former Lebanese Army Maj. Saad Haddad.

[Washington Post correspondent William Claiborne reported early Monday that the Israeli Army said it has taken Beaufort.]

What is important is that after nearly eight years of constant violence and occasionally serious warfare, the Lebanese be convinced that the Israelis do have a concrete objective and a plan to make it work.

In a way such thinking is a measure of Lebanese hopelessness.

For even if the Israelis do not have a plan, the Lebanese are so near their wits' end that they are more in a mood to blame their fellow Arabs than the Israelis for turning their country into the central Middle East battle zone.

For since the 1978 invasion of Lebanon, the Lebanese have grown ever more cynical about the Arab world in general and their Syrian and Palestinian "guests" in particular.

To many Lebanese the invasion today was a rerun of 1978, but without the hesitations and strategic errors that the Israelis long since have conceded in public marred that operation.

Then, the Israelis moved ahead and stopped, unclear about their goals and suddenly spurred into a renewed offensive when to their surprise then-president Jimmy Carter mounted a major diplomatic offensive to condemn them and get them out of Lebanon.

That effort failed in part, as evidenced by the continued Israeli military presence in Haddad's zone--despite U.N. Security Council resolutions that the 7,000-man U.N. peace-keeping force move down to the international border between Israel and Lebanon.

This time the Israelis have moved forcefully across the Litani River, set up blocking forces, cut lines of communication and borne down on their Palestinian foe.

The Israelis did so assuming, apparently correctly, that the Arab world would acquiesce in the invasion. For never in the 36-year-old history of the state of Israel has the Arab world been in such disarray.

Hardly a day goes by but that Lebanese Moslems, both Shiite and Sunnis, who once celebrated the Palestinians as Islam's beau ideal, are involved in some shooting fray with their erstwhile heroes.

Syria, which is in principle sworn to defend the Palestinian interests, has had its own shootouts in Lebanon with Lebanese Moslems backed by the guerrillas.

Indeed, the citizens of this half-Christian, half-Moslem nation have become so fragmented and discouraged that they have little use for the Syrians.

History records that they counted on Syria to save them in their hours of need during the Lebanese civil war in 1975 and 1976.

Thus, just as the Israelis gambled correctly on the Syrians' staying out of the 1978 fighting, much to the impotent fury of the Palestinians' allies, today that gamble appears still without risk.

Palestinians and Syrians have killed each other recently in a series of fights in the northern Lebanese city of Tripoli.

Today, the Syrians' main concern is unseating their rival Baath Party government of President Saddam Hussein in Baghdad.

Never before have the Palestinians been so naked.