Now that its military superiority in the region has been demonstrated beyond question in the bleak hills of Lebanon, Israel has emerged as a force to be reckoned with in an entirely new light by both the United States and the Soviet Union.

For the United States, Israel's overrunning of nearly half of Lebanon in less than a week raises a serious question of whether the Israeli Army's lightning advance has put the government of Prime Minister Menachem Begin, or whoever succeeds him, beyond the reach of U.S. leverage for the rest of the decade.

The sheer size of the invasion force and its complete dominance as it crushed with awesome technological advantage both a vastly improved conventional military force of the Palestine Liberation Organization and a well-equipped Syrian Army that long had been expecting an attack, would seem to add a new dimension to Israel's capacity to resist American pressure whenever U.S. and Israeli interests diverge.

With an arsenal of U.S.-built weapons whose effectiveness was stunningly revealed, Israel appears almost immune--for several years at least--to an arms cutoff or postponement of weapons delivery by the Reagan administration. Some American critics have called for such a cutoff in retaliation for the invasion into Lebanon.

In the diminishing likelihood that it has to go to war with a seemingly immobilized Syria, or an increasingly reluctant Jordan, or an Iraq distracted by its own conflict with Iran, Israel could dip into an arms stockpile more potent than any of its antagonists appear to have realized. Moreover, now at peace with Egypt, Israel for the first time in its 34-year history does not face the certainty of a challenge from the south, in the event of a war.

For the Soviet Union, Israel's dramatic victories over a heavily armed client state and an attentively backed liberation movement would suggest that David has outgrown Goliath and that it may no longer be advisable for Syria--or, indeed, any Arab state--to throw down the gauntlet.

The humiliating defeat of Syria's Soviet-equipped Air Force, the eradication of the Soviet-built Syrian surface-to-air missile system and the damage done to the Syrian Army's Soviet-built tanks even raises questions of how well a limited deployment of Soviet forces would do against the Israeli defense forces if Moscow were to decide that Syria needed more direct assistance than the supplying of arms and military advisers.

It seems self-evident that Israel will now have more difficulty casting itself in the image of the underdog in the Middle East, hemmed in and beleaguered by an array of Arab states bent on its destruction. The Arab states may still be bent on Israel's destruction, but after the war in Lebanon, the notion of constant vigilance to avoid military inferiority will be a harder commodity for Israel's image makers to sell abroad.

At home, the invasion of Lebanon has had far-reaching implications for Israel's self-confidence, which was badly damaged by early losses against the Egyptians and Syrians in the 1973 war. Begin said last week that the successes in Lebanon had "healed the nation of the Yom Kippur war trauma," adding, "I am absolutely certain that, with God's help, we will stand up to all our enemies and overcome them."

Israeli officials have tried to avoid gloating publicly over the Soviet Union's indirect losses on the battlefields of Lebanon and have been careful to point out, for example, the shortcomings of the Syrian tank crews, the more advanced training of Israeli pilots and the high performance of U.S.-supplied electronic warfare equipment.

"This operation was not intended to test the effectiveness of Soviet weaponry or to challenge Soviet interests in the Middle East. This may have been a side effect of what transpired, but that was not our purpose," one Israeli official said.

But another senior official, in a moment of euphoria, revealed Israel's perception of a diminished Soviet influence in the Middle East resulting from the Israeli successes.

"Who runs the show in the Middle East now?" he asked rhetorically in the eighth day of the war. "The answer is, the United States and Israel."

If the Reagan administration steps up its assertiveness in the region, he added, Soviet influence in the Middle East could be diminished even more.

The lopsidedness of the war in Lebanon was obvious at the end of the first week.

The PLO had been reduced to a military eunuch, 5,000 to 6,000 of its fighters captured by the Israelis in southern Lebanon, uncounted more killed and another 5,000 trapped in West Beirut by a steadily tightening circle of troops from the Israeli Army and the Christian Phalange forces.

The Syrian armed forces were battered in the east and isolated in the west by the relentless Israeli advance. By Israeli count, Syria lost 85 front-line aircraft, 30 surface-to-air missile batteries, 350 tanks and numerous combat units were either in disarray or redeployed to safety.

Among the Syrian tanks lost were 10 Soviet-made T72s, making their first battlefield appearance. Regarded as the most advanced Soviet tank, the T72s were knocked out by Israel's own Merkava in a test that immediately piqued the interest of NATO and Warsaw Pact forces.

With its headquarters in Shura leveled and its tactical advantage lost by the end of the week, the 23,000-strong Syrian force in Lebanon pulled back from the Israelis and limited itself to occasional and desultory artillery exchanges.

Also, Israel, which in the 1973 Middle East war lost scores of fighters to Soviet-built Egyptian and Syrian surface-to-air missiles, demonstrated that it had refined its electronic countermeasures enough to paralyze Syria's air defense system and master the skies of Lebanon.

The war also revealed a major strategic innovation in the Israeli arsenal--the effective use of large-scale seaborne armored units. Logistically complicated and potentially hazardous, the seaborne armored force landed on the Mediterranean coast and quickly captured the PLO strongholds of Tyre and Sidon.

The impressive performance of Israel's Air Force, which reported losing only one jet fighter, so intimidated the Syrians that they stopped sending Mig fighters to intercept the U.S.-supplied aircraft. U.S. Defense Department officials presumably are eager to talk to the Israeli pilots about what lessons are to be learned on the use of the F16s and other top-performance fighters in their first sustained aerial combat.

Begin and other Israeli leaders have long complained about the United States' role in arming Arab adversaries, saying that Israel's disadvantage in quantitative terms was growing steadily. Begin repeatedly has said there is a point at which quantitative superiority in arms can become qualitative, and that the sale of sophisticated weapons to the Arab states renders even more meaningless any qualitative arms edge that Israel may have had.

The results of the war in Lebanon would suggest, however, that Begin underrated the practical effectiveness of Israel's qualitative edge.

As if awed by the Israeli advance over the Syrian Army, the Soviets' reponse has appeared low key. In all of Israel's previous wars--indeed, in many limited operations against the PLO--the Soviet Union would make apocalyptic pronouncements against Israel, and the Soviet fleet often made threatening moves.

Yet now, while the PLO, an integral part of Soviet diplomacy in the Middle East, is being systematically dismantled, the Soviets have demonstrated less support for the Syrians than during other conflicts, and arms supplies from the Soviet Union to Syria have not been at a level great enough to alarm the Israelis.

One explanation could be that the Soviet Union has always respected force in the application of its own foreign policy, and that Israel, having demonstrated that it is the dominant power in the region, has forced Moscow to reevaluate its policy in the Middle East.