That the fight over the sale of anti-aircraft missiles to Saudi Arabia comes at the moment when Iranian forces seem poised to break through Iraqi defenses is surely a coincidence. That the pro-Israeli lobby in Congress is leading the fight against the sale is pure Pavlovian reflex.

For as long as one can remember, the American Israel Public Affairs Committee has ferociously spearheaded opposition to the sale of arms to Arab countries -- any country, any kind of arms. To do so, under our system, is surely its right. The exercise of that right, however, is not in Israel's interest, or America's.

The open question in dealing with the Saudis is not whether they will use their arms against Israel, but whether they are prepared to use them to defend themselves and their neighbors.

The Saudis know from whence the danger comes, and it does not come from across the desert in Zion. The enemy is Iran, across the water to the east. Iran is a culture hostile to the Arabs; its people are Shi'ites, a rival branch of Islam; its society is medieval, aspiring to establish a modern-day theocratic empire. The prospect grows more menacing daily.

Though poorly fed and ill-equipped, Iran's troops, according to the evidence from the battlefields to the north, are driven by a zealotry that the Iraqis simply cannot match. Iraqis have been promised a better life by their government. Iranians have been promised a better death, and they surge relentlessly forward in the face of machine guns, tanks, even poison gas.

Strategists do not know whether Iran, having largely overrun Iraq's defenses where they meet the border of Kuwait, is planning to turn next toward Baghdad or south into the Arabian penisula. The region is watching the battle with apprehension.

The Saudis have no tradition of defending Arabia from outsiders. They are a desert people who still think in terms of tribal confrontation. Though never colonized, they let Britain defend them as long as the empire lasted, and, afterward, they accepted Washington's advice to rely on the shah. Now the shah is gone, the United States has not filled the gap, and the shah's successor makes no secret of a desire to swallow them up.

At the start of the Gulf war, the Saudis were roused from their insularity to organize the neighboring principalities -- Kuwait, Qatar, Bahrain, Oman, United Arab Emirates -- into a loose alliance called the Gulf Cooperation Council. To avoid offending Iran, the GCC said its goals were economic, but it is the Saudis' opening effort to confront the need for collective security in the region.

The Iran-Iraq war also pushed them to embark on an expensive program of defense -- but of defense alone. Their early warning system is first-class. But the regime remains essentially inward-looking; it fears that if it establishes a strong army, the monarchy may find itself challenged. The dilemma for Western interests is not whether the Saudis will attack Israel, but whether they will fight for their homeland and their neighbors.

One need not be a strategic genius to recognize that the threat to Israeli security from the east is not from lethargic Saudis, even less from the much-bloodied Iraqis. The danger is heavily populated, frenetically motivated Iran. Does anyone doubt that the road from Tehran to Tel Aviv passes through Baghdad, barring a detour through Kuwait and Riyadh?

Yet old habits among Capitol Hill lobbyists -- including the American Israel Public Affairs Committee -- die hard. The lobby is more anxious than Israel itself to stop the missile sale. Part of the explanation is that Israel's coalition government has competing foreign policies -- the official policy of the prime minister and the hard-line policy of the foreign ministry. The lobby's ties, traditionally, are with the foreign ministry.

Lobbies also tend to acquire vested interests of their own, apart from those of their clients. Thus the meas the sale to the Saudis becomes its own image of invincibility. As much as anything, however, the explanation is habit. Organized for a certain job, a lobby does it automatically.

Israel scarcely needs to adopt the Arab maxim that "the enemy of my enemy is my friend." Saudi Arabia is not Israel's friend. But the ayatollah's Iran is, and is likely to remain, a more menacing enemy for some time. Israel must set priorities, and the first is to stop Tehran from establishing domination over the Persian Gulf. Its friends would do well to help it.