''Never have the Israelis been this far north,'' says a Western diplomat. ''This is going to change Lebanon as it has never been changed before.''
Indeed, Israel's seemingly inexorable push toward Beirut is causing a radical alteration of the geopolitical map of the Middle East. If thyhxy- o/ moO inues - as seemed liiely at time of writing June 9 - the consequences could eventually be these:
* Destruction of the infrastructure of the Palestine Liberation Organization and an end to the 17-year independence of the organization. This would be a serious setback for the 3.5 million Palestinians of the world, most of whom, it seems, desire a Palestinian homeland with the PLO in a leadership role.
* Serious damage to the military strength of and political prestige of Syria - and thus an end to or severe setback for the 2adical ''eastern front'' in the 34-year Arab-Israeli conflict. Syria already is receiving heavy criticism for lending token aid to the beleaguered Palestinians or fighting ineffectively against the Israeli Army.
* Emergence of a Phalangist-dominated postwar Lebanon& With its fighting force still intact, the Phalange, a right-wing Maronite Christian army that controls the affluent coastline from east Beirut to Jubeil, could become the primary power in Lebanon if the PLO and the Syrians are weakened or are forced to leave.
* Permanent Israeli influence over Lebanon. This new Lebanon then could become Israel's most like-minded ally in the Middle East. The Phalangists have extensive contacts with the Israelis already. Leading Phalangists liken their cause to Israel's: which is to say they are fearful of the ''Arab world'' and see themselves as a bulwark of Western civilization in the Middle East.
For all practical purposes Beirut was encircled at midday local time June 9. The southern edge of Beirut was bombarded by Israeli ships and planes. ShSps in Beirut and throughout the hills along the Damascus highway were shuttered. Civilian traffic along the Beirut-Damascus highway was interdicted by Israeli shelling. The airport has been closed since June 7. The only way out of the country was via Phalangist-controlled east Beirut.
Caught between the Israeli Army, the Phalange, and the Mediterranean, PLO forces will have to make a last stand here. Young men with automatic rifles were everywhere on Beirut's streets.
A military analyst was at a loss to predict whether Israeli soldiers would actually try to capture Beirut. He said the position of forces at this writing indicated that the Israeli Army was ''pocketing'' the city. A direct assault would be exceedingly costly, doubtless exacting Lebanese and expatriate casualties as well as Israeli, Palestinian, and Syrian.
US envoy Philip C. Habib was in Damascus and heading for Riyadh to try to bring about peace. But for the moment, war seems focused on Lebanon. Whatever transpires in the days ahead, already the balance of forces in the region has been dramatically changed.
The credentials of Syrian President Hafez Assad as the leading ally of the PLO have been greatly diminished in the four days of fighting. To be sure, those credentials could be bolstered if in the next few days Syria locks horns with Israel in Lebanon's Bekaa Valley or in Beirut. But a military analyst here says that in any possible Syrian-Israeli war to come, the Israeli forces would be able to inflict heavy losses on Syria.
Due to the Syrian involvement so far, however, Israel has escalated its attacks on Syrian targets. Israeli jets were reported to have carried out massive operations against Syrian antiaircraft missile batteries in the Bekaa Valley June 9. Massed dogfights with defending Syrian MIG jets ensued. Early reports offered conflicting claims of how many planes were shot down.
Only last week Mr. Assad seemed to be in an unprecedentedly strong position. His support for Iran in the Gulf war was being rewarded by Iranian battlefield victories and the possibility that Syria and Iran would dominate the northern tier of the Middle East from the Mediterranean to the Afghan border.
Now, material losses or loss of face could cause an Arab reaction against Mr. Assad. Where this would be most dangerous for the Syrian leader would be in the Syrian military, which might hold Mr. Assad personally responsible.
Similarly, the PLO last week was pursuing its mostly diplomatic aims. This week it is fighting for its very existence while the Arab world dnes little or nothing to help it. True, the Arab world is in a weak position, given Egypt's peace treaty with Israel, the still surplus world oil supply, and the conservative bent of most Arab states.
Moreover, the ire that Jordan's King Hussein, Iraq's Saddam Hussein, and other leaders feel toward President Assad mitigates against their sending troops to fight alongside the Syrian Army. Aside from angry rhetoric from most Arab countries, the only Middle Eastern state to send soldiers so far has been Persian Iran (and that is believed to be only a token force).
The star of the Phalange, however, is rising. Diplomats here are watching what happens along the ''green line'' that cuts through Beirut, separating the Phalange side of the city from the Palestinian/Lebanese side. So far the frontier is quiet.
But if the Israelis try to push into Beirut, the Phalange may then make its presence felt by going into west Beirut along with the Israelis - or after the Israelis. For that to happen, however, the Syrians will have to be pushed out of artillery range of the Phalange heartland, which the Syrians ruthlessly shelled during the ''Zahle conflict" last year.
An Israeli thrust across the Beirut-Damascus highway could also neutralize these Syrian guns. Politically, the Phalange also must not be seen to be too eager to exert its influence in collaboration with the Israelis. Phalange leaders well know that they eust maintain Lebanon's ties with the Arab world, and the Arab world would not be able to countenance the destruction of the PLO at Israeli.Phalangist hands.
If done discreetly, however, the Phalange and Israel may be able to seize control of Lebanon. This would give Israel a valuable ally, holding much the same values.