One Sunday last October, a PLO brigade with tanks and artillery staged a mock attack on a Lebanese settlement resembling a fortified Israeli village on the West Bank. This training exercise was evidence of the accelerating evolution of the Palestine Liberation Organization in southern Lebanon into a quasi-state with a conventional army of 15,000, and logistical support from 23,000 Syrians.

The PLO benefited from U.S. diplomacy last spring when Syria put missiles in Lebanon. Only a patch of bad weather prevented Israel from promptly destroying them.

Such a shot across the bow of the PLO and Syrian occupation forces in Lebanon might have slowed the slide toward today's crisis. But by the time the weather cleared, President Reagan had sent an entreating letter, soon followed by a special ambassador, to paralyze Israel. Did the Reagan administration expect to talk the PLO into tranquillity? Perhaps the administration's Saudi Arabia enthusiasts expected Saudi "leverage" on the PLO--leverage supposedly resulting from Saudi financial support--to be used for moderation. But Saudi payments to the PLO are protection payments from the timid to the ruthless.

Israel repeatedly told U.S. officials it could not tolerate on its border a quasi-state that is terrorist in morality and form. Today Israel's primary aim--an aim that serves U.S. interests --is more ambitious than the stated aim of pushing PLO artillery too far north to hit Galilee. It is to drive 15,000 PLO members off the land they have usurped for bases and training. The PLO, a state without a nation, would become an army without an infrastructure. This would leave them with no place to go, and a powerful incentive to melt away.

Israelis believe that any PLO attempt to relocate in northern Lebanon would kindle a salutary Lebanese nationalism, sparked by resentment of PLO and Syrian occupation.

The existing Palestinian state is Jordan. It has a Palestinian majority, and a majority of all Palestinians hold Jordanian citizenship. But it will not receive the PLO. Jordan could have established a separate Palestinian state on the West Bank during the 19 years it held that land. Jordan never considered it. And in "Black September," 1970, Jordan expelled the PLO, bloodily.

Might Syria's rhetorical solidarity with the PLO extend to allowing the PLO to locate in Syria? Not likely. Syria engaged Israel's forces to protect its own forces, not to aid the stricken PLO.

The disorganization and demoralization of a dispersed PLO should diminish the terrorist organization's diplomatic weight in the world's capitals. It also might reduce PLO intimidation that has impeded West Bank accommodation.

Only the Soviet Union has an interest in a strong PLO. But many European governments, and elements of the U.S. foreign policy establishment, have invested a lot of ingenuity in rationalizing appeasement of the PLO. They have treated the PLO as a state governing a diaspora, and the sole legitimate representative of Palestinians. Israel's eviction of the PLO has been made especially necessary by Western diplomacy that has sanitized the terrorist organization, and thus emboldened it.

If Israel succeeds in creating a political vacuum in the portion of Lebanon the PLO controlled, the urgent task, for others as well as Israel, will be to fill the vacuum. There are three basic options. Two seem improbable; the third presupposes a rationality and coherence rare among indigenous Lebanese forces.

One option is for the United Nations to expand its peacekeeping presence. But the United Nations, a plaything of its anti-Israel majority, is not apt to adopt a plan so compatible with Israel's purposes.

A second option is a peacekeeping machinery constructed outside U.N. auspices. But who will do it? France has historical ties to Lebanon, but is not apt to initiate a solution. No first or third volunteers spring to mind.

The third option is to stitch together a fabric of Lebanese sovereignty. Nation-building is a dicey business at the best of times. It is staggeringly so when the society is as invertebrate as Lebanon's is now. But the place to begin is with the central government, such as it is, and the Christians, the most cohesive indigenous force. The Christians have an ideology, a political party and a considerable military command.

The splotchy map of Lebanon--a Jackson Pollock canvas of PLO and Syrian units-- must have told Israel's experienced soldiers that they could not operate against PLO units without engaging Syrian units. Evicting both would serve Israel's purpose by enabling northern and southern Christian elements to join. As has been said, the West should support Lebanon's Christians, or the West should change its name.