Philip Habib is scheduled to resume this weekened his efforts for President Reagan to keep the Lebanon crisis from exploding into another Middle East war. His first round of shuttle diplomacy to that end lasted from May 7 to 27 and succeeded in achieving its main American purpose -- to dissuade Israeli Prime Minister Menachem Begin from starting another round of shooting against Syria.
This second Habib round will try to go a little further. The first round secured a temporary truce, but no more. The second round must try to reduce the underlying tensions between Israel and Syria.
The key to the success or failure of this second diplomatic round by Washington's special negotiator for the Middle East is in Mr. Habib's pocket. How much authority has President Reagan given him to restrain Israel's creeping political-military penetration of Lebanon?
The present crisis is usually presented in terms of the deployment of Syrian surface-to-air missiles inside Lebanon. That happened on April 29, one day after Israeli fighters shot down two Syrian helicopters over the Bekaa Valley in eastern Lebanon. Mr. Begin promptly demanded removal of the missiles (Soviet-built SAM-6 weapons) and has ever since insisted that unless they are removed by the Syrians, he will send the Israeli Air Force in to destroy them.
But the shooting down of the two Syrian helicopters, followed by the Syrian deployment of surface-to-air missiles, is only an incident in an older and continuing story.
Syria has always felt it should have all or much of Lebanon.
Recent publication of Israeli memoirs shows that as far back as the 1950s Israeli leaders were thinking in terms of using Maronite Christians in Lebanon as instruments for their purposes. This concept has long since been put into action.
Militia units, recruited largely from among Maronites, have taken over a strip of southern Lebanon lying just north of the official Israeli frontier. The area is now known as Haddadland. It is policed by forces under the command of a Maj. Saad Haddad. They are largely paid and armed by Israel. Haddadland has become a de facto Israeli military possession.
Beginning last December, members of another Lebanese militia unit, also largely recruited from among Maronite Christians, went into action. They call themselves Phalangists. Their base is at Juniye, a seaport north of Beirut which is the de facto capital of a Maronite enclave in central Lebanon. In December Phalangist troops had crossed the crest of the Lebanon range and moved down into the largely Greek Orthodox city of Zahle, which lies in the Bekaa Valley.
The Syrians have deployed their troops in much of the Bekaa Valley since 1976 as part of the Arab Deterrent Force intended to stop the Lebanese civil war.
There was fighting around Zahle in December. A truce was patched up on dec. 26 and lasted more or less until April 2. Between Dec. 26 and April 2 President Reagan took office in Washington, having during his political campaign said all the things Israeli hawks wanted him to say, including that Washington regarded Israel as its most important ally in the Middle East. His new White House foreign policy adviser, Richard Allen, made a speech that said Israeli operations in Lebanon partook of the nature of "hot pursuit."
Whether President Reagan intended his own and Mr. Allen's words as a green light for Israel to act with less restraint in Lebanon, the result has been just that. Israeli military operations have been stepped up steadily ever since.
Israeli chief of staff Lt. Gen. Rafael Eitan was reported to have visited the Juniye headquarters of the Phalangists in late March or early April. Phalangist operations in and around Zahle increased. The Syrians retaliated beginning on April 2. There has been trouble around Zahle ever since.
Israel has not continued the printed London report on April 13 about the alleged Eitan visit to Juniye. But Israeli military authorities have confirmed publicly that the Phalangists were being supported and armed from Israel.
So as Mr. Habib returns to his rounds of talks in the capitals of the Middle East, the real question is whether he will be able to use US influence to restrain Israel's steady extension of military intervention and control in Lebenon. If Israel is to use the Phalangists as one instrument for trying to drive the Syrians out of Lebanon, while at the same time waging heavier war on Palestinian forces in the part of Lebanon lyying between the two Maronite militia forces, there is not much hope for any easing of the underlying strain.
The basic situation is that if Syria were to give in to Israeli tactics and withdraw its forces from Lebanon, Israel would enjoy something like actual military control over all of southern and central Lebanon.
Any such Syrian withdrawal with such consequences is hardly to be expected. Syria will certainly fight to retain control of the Bekaa Valley, which leads north to Homs -- a major Syrian city and the hub of its communications system. Besides, resistance to Israeli penetration of Lebanon would be popular throughout Arabia and would help to restore Syrian prestige among Arabs. that prestige was tarnished when Syria supported Iran against Arab Iraq.
Israeli moves into Lebanon were restrained consistently during the Carter administration. Israel has been behaving since then as though it had been unleashed by Washington. Israeli attacks on suspected Palestine Liberation Organization centers in Lebanon have been heavier than ever in recent days. Since the United States provides Israel with most of its major weapons and with vital economic and financial support, Washington has considerable leverage to restrain or release as it chooses.
There have been hints that President Reagan has felt that Mr. Begin was going too far and too fast. But as of this writing there is no public knowledge of the instruction Mr. Habib is carrying with him as he picks up the threads of his Middle East diplomacy.
Mr. Habib's problem is complicated by the further fact that Syria has its own ambitions in Lebanon. It would perhaps be willing to withdraw entirely from Lebanon, though reluctantly, provided Israel would also pull out and abandon its use of the Maronite militia units.
But Israel feels that it must for its own security against guerrilla attack push the Palestinian forces now refugeeing in Lebanon farther north and out of reach of Israel's own frontiers. Syria will certainly want to control as much of Lebanon as Israel will. It particularly wants the fertile Bekaa Valley, which parallels Syria's own frontier and would give Syria some protection against a possible Israeli attack.
Mr. Habib will need to be as skillful as Henry Kissinger ever was to make much headway among these conflicting interests -- unless he has as much power to restrain Israel as Mr. Kissinger did during his Middle East tours.