IN 1840 BRITAIN'S prime minister, Lord Palmerston, wrote that Syria is the key to the Middle East, and Lebanon is the key to Syria. How smug Palmerston must feel now, looking down at his modern day counterparts who have yet to figure this much out. Nearly 150 years later, Palmerston's dictum is finding its partial realization in the new Israeli- Lebanese security agreement. But in reality this bargain made by Secretary of State George Shultz is only half a loaf. And if Syria holds out too long, it will become moldy.

The imperatives of Syria and the world community which desires peace are diametrically opposed. The priority of Syrian President Hafez Assad is self-preservation. Assad, who stays in power by externalizing the threat, is already a legend in survival. A member of the minority Alawite sect in a country with an overwhelming majority of Sunni Moslems and with a history of semi-annual coups, his cunning has kept him in power for over a decade.

If Syrian troops ever left Lebanon, Syria's social, economic and religious differences would take center stage, threatening Assad's power. For example, if Lebanon were dropped from the Syrian agenda, the Syrian Sunnite majority might begin to make noises about Assad's allocation of more than 50 percent of the nation's buget on the northern Alawite provinces.

In addition to diverting attention from internal disorders, Assad maintains his longevity by his success in creating a Syrian valse extraordinaire and in taking turns in slow and intimate dancing with the Soviet Union and United States. He has managed to both consummate a strategic agreement with Russia and, at the same time, raise American hopes that he may still bring Syria back into the Western orbit.

Despite the presence of Soviet advisers and massive shipments of Soviet equipment to Syria, Some American officials have regarded Syria as a factor of stabilization in Lebanon and the Middle East. In 1976, President Carter's special envoy, Dean Brown, told the leaders of the Lebanese resistence that the Syrians were the new equivalent of the U.S. Marines in 1958 -- in other words, a peacekeeping force.

Even now, the Reagan administration is willing to bend to Syrian blackmail. The Saudi military attache in Washington, Emir Bandar, recently met with Shultz to discuss negotiations between Saudi Arabia and Syria over what price the Syrians are asking just to sit down and talk to Lebanon about a troop withdrawal. These talks have the official imprimatur of the United States. Arab sources confirm that two issues are under discussion. Syria wants $4 billion dollars up front and Saudi guarantees to crush the opposition of the Moslem Brotherhood to President Assad. So far, as temporal leaders of the Moslem faith, the Saudis have been unwilling to do this.

The Arab world must stand up and be counted on the issue of peace. Previously, the ambiguities in the Arab position toward relations with Israel effectively postponed final decisions. Now the Israeli-Lebanese agreement is forcing the Arab world to take clear-cut positions. And it has become obvious that the only obstacle to that step is Syria.

Why does Syria have this power in the Arab world? The reason is that the fragile Arab regimes, not based on popular will, are afraid of Syria's threats to topple them from within and without. Syria's willingness to back the non-Arab Iran against Arab Iraq demonstrates Syria's inclination to punish any regime that falls from the Syrian orbit. Another example is Syria's current effort to topple Yasser Arafat and become the sole voice of the Palestinean people.

In this situation the United States has three options. The first is to give up. This is not only politically unacceptable to the Reagan administration, but it would also create a vacuum to be filled, presumably, by the Soviets. The second is to get a Lebanese-Syrian agreement at any cost -- sacrificing Lebanese sovereignty by imposing such an agreement. This will not bring lasting peace. The third is to free the Arab world from the Syrian veto on further diplomatic progress.

If this could be done, the benefits would be great. Once freed from Syrian threats, the Arab would realize, after five wars and five defeats, that it must deal with Israel. The support from the majority of the Arab states for the Israeli-Lebanese agreement demonstrates this.

But as long as Syria can delay peace, these same Arab states will back off and knuckle under to Syria. Kuwait is the newest example. In recent days it has become the first Arab state to back off from a prior approval of the Israeli-Lebanese agreement. The longer Syria has, the morestates it will be able to intimidate.

On the other hand, freeing the Arab states from the Syrian thumb would enable Lebanon to fulfill its historical role of bridging the differences between cultures and interests. Even if the Arab states accorded full recognition to Israel, major problems of acceptance of Israel by the Arab world would remain. A leavening role between Arab and Jew is needed; this role could easily be filled by the half million Lebanese expatriates currently in the Middle East. They are fully integrated into the Arab world, they are highly skilled managers and supervisors who have proved adaptable to politics and economics.

But if it would be desirable to free the Arab world from Syrian hegemony, it will not be easy. Perhaps only the fall of Assad or another war will create the necessary preconditions.

What if Syria were partioned into mini-states dominated, respectively, by the Alawite and Sunni factions that now coexist uneasily under Assad's rule? In such an arrangement, the interests of both the Palestinians and the Soviets could be served. The Palestinians might get what amounted to a state in the area bordering Jordan, and the Soviets will have an assured, continued presence in both halves of a partioned Syria, without the uncertainties that go with the current arrangements.

The international community established a precedent for partition of this kind in Germany after World War II. Just as a united Germany then seemed to pose an unacceptable threat, so a Syria united under Assad today may be an unacceptable member of the community of nations.