An intense national debate has begun in Israel over whether Prime Minister Menachem Begin should offer concessions to Syria to extricate Israel from the crisis over the antiaircraft missiles in Lebanon, or risk a full-scale Middle East war by remaining adamant about national security objectives.

The Israeli consciousness seems to have been numbed by the rush of events of the last two weeks, and the extraordinary openness in which both sides have maneuvered themselves into the most ominous and intractable showdown since the 1967 war.

Ezer Weizman, the former defense minister, likened it to two enemies climbing a very tall tree who -- finding it impossible to climb down -- lock themselves aloft in mortal combat until both fall.

For five years, Israel has managed to maintain with Syria a delicate balance of policy objectives in Lebanon, both sides walking a thin line with an unsigned gentlemen's agreement, which for the most part had been studiously adhered to by both parties.

The Syrian Army entered Lebanon in 1976 to end the civil war that had wracked the country for two years and quickly received an Arab League endorsement of this goal. At the time, Israel agreed to tolerate the intervention on the condition that the Syrians limited themselves to policing the warring factions.

The terms were spelled out in a letter sent in March 1976 by then-foreign minister Yigael Allon to Henry Kissinger, who had asked the Israelis to define their vision of a Syrian "red line" in Lebanon. The letter was passed by Kissinger to the Syrians, and it was Israel's understanding that the terms were tacitly accepted.

According to the Israeli interpretation, there was to be no interference by Syria in Israeli ground operations or air strikes against Palestinian guerrilla positions, particularly those south of the Zaharani River; no Syrian naval operations along the Lebanese coast; no movement of Syrian troops south of the Zaharani; no more than one infantry brigade south of the Beirut-Damascus line; no deployment of Syrian missiles in Lebanon; no Syrian attempt to close the Christian port of Jouniyeh, and no imbalance in the Syrian moves against the Christians and Palestinians.

It is that "red line" agreement to which Begin refers when he demands that Syria return to the "status quo ante" that existed until a month ago.

It was then that the Syrians, apparently provoked by Christian attacks on the Bardouni River bridge in the strategic Bekaa Valley northeast of Beirut, moved to take Mount Sanin overlooking the Christian city of Zahle. Syria regards the Bekaa Valley as its "soft underbelly," and President Hafez Assad has made no attempt to disguise his determination to control the valley.

The Israeli Air Force responded to the Syrian offensive on April 28 by shooting down two Syrian helicopters near Mount Sanin, and the Syrians the next day deployed the first three of the 14 surface-to-air missile batteries now located near Lebanon or on the Syrian side of the border. Five of the batteries are on Lebanese soil, according to Begin.

Begin's initial reaction to the Syrian offensive against the Christians was to invoke what he called Israel's "sacred moral commitment to prevent the annihilation of the Christians of Lebanon."

But it became clear in his subsequent remarks on the subject that Begin's concern extended beyond the safety of the Christians, to the much larger question of Israel's national security.

As he indicated in his speech to parliament Monday, Begin is acutely aware that the Christian forces are tying up a large part of Syria's 30,000 troops in Lebanon, and that an unencumbered Syrian Army might soon exert its control over all of Lebanon.

"Whoever controls the Sanin mountains and the skies of Lebanon controls all of Lebanon up to the Israeli border," Begin said, "There is no doubt about that. And then, Israel will face a threat to its existence. Then, war will be inevitable under the worst possible conditions for Israel." t

Another key element in Israel's decision to risk war with Syria over its actions in central Lebanon is its heavy dependence on preemptive air strikes as a deterrent to terrorist attacks launched from Palestinian bases in Lebanon against civilian targets in Israel.

Since Weizman announced in 1979 a policy that Israel would not merely retaliate for terrorist raids, but would continuously conduct bombing raids against guerrilla training camps, cross-border infiltration into Israel has decreased dramatically.

Without this freedom of movement in Lebanese skies, Begin said, the Air Force would be ineffective against the guerrillas, and the Palestinian training camps would be reestablished on a permanent basis, leading to increased terrorism in Israel.

Having committed himself so absolutely and publicly to each of these policies as integral parts of Israel's overall security, Begin does not appear likely to compromise on any one of them, particularly on Israel's right to conduct preemptive strikes without the danger of Israeli pilots being downed by Syrian antiaircraft missiles.

Indeed, even the opposition Labor Party, which has bitterly condemned Begin's handling of the crisis, agrees that there is a national consensus in Israel on the necessity of removing the missiles.

But Labor Party leader Shimon Peres and other party spokesmen say there is no consensus on Israel's obligation to rush to the defense of Lebanese Christian forces so far from Israel's border.

The problem for Israel has become one of having to choose which, if any, of its policies in Lebanon it could and should forfeit if it becomes necessary to make a concession.

Peres and his Labor supporters seem eager to return to the policy of "helping the Christian forces help themselves," and possibly giving Syria assurances that Israel will not intervene as it did on April 28 in the Bekaa Valley. In return, the missiles would be removed, and Israel would be free to conduct preemptive strikes against the Palestinian bases and make aerial photo reconnaissance flights.

The debate, both in public and in private consultations with U.S. special envoy Philip C. Habib, has just begun, and it remains to be seen whether any way can be found to ease the crisis.

But in any case, it appears certain that success cannot be attained unless the 1976 "red line" agreement -- abrogated by both Syria and Israel -- is restored in one form or another.