Menachem Begin's free hand

The fortunes and prospects of the Arabs of Palestine have reached an all-time low. Their only remaining hope for restraint on Israeli expansionism is in President Reagan's hands in Washington. But when he suggested this past week that he would ''guarantee'' Israel's northern border if Israel would take its troops out of Lebanon, the idea was brushed aside as meaningless.

Israel is not interested in an American ''guarantee,'' said Israeli Foreign Minister Yitzhak Shamir. The Israelis want ''an agreement between us and Lebanon.'' The agreement they are asking for would give Israel continued control over southern Lebanon.

The diplomatic agenda calls for Israeli withdrawal from Lebanon first, then talks about ''resolution of the Palestinian problem.'' Israeli troops have been in southern Lebanon since last June. Mr. Reagan has been trying to talk them back since last September. His talk has not budged them or done anything for the Arabs of the occupied West Bank, Gaza, east Jerusalem, or the Golan Heights.

Camp David held out for Palestinian Arabs the prospect of political ''self-determination.'' That prospect was refreshed in President Reagan's proposal of last September. But six months have gone by during which the President could not even get the Israelis out of Lebanon or halt the construction of Israeli housing in the West Bank.

The past week brought other depressing news for the Palestinian Arabs. Saudi Arabia and the Arabs of the Gulf states once had leverage in Washington through oil. With the near collapse of OPEC and falling oil prices that leverage is fading fast.

The Palestine Liberation Organization (PLO) once could bring some help to the Palestinian Arabs. But its leaders are scattered and fragmented. They met this past week in Algiers and disagreed over basic diplomatic tactics. It was largely an exercise in futility. Some wanted to endorse President Reagan's peace plan. Others opposed. It made little real difference.

One of the few active friends the Palestinians have is Col. Muammar Qaddafi of Libya. The colonel apparently has been trying to engineer a coup d'etat in the Sudan. It fizzled. As much as Colonel Qaddafi wants to help the Palestinian Arabs, he is incapable of doing anything effective. He failed to help the PLO armed forces in Lebanon. He can't even pull off a successful coup d'etat in the Sudan.

In theory, Iraq and Syria should be helping their Arab brothers in Palestine. But Iraq has its hands full with its war with Iran. Besides, Iraq and Syria dislike each other so intensely that they would probably be incapable of coordinating their efforts even if there were no war with Iran. And Syria's armed forces were so badly mauled by the Israelis in last year's fighting that they dare do nothing to provoke an Israeli attack.

In other words the Arab community is fragmented, incapable of shaping and pursuing a common strategy against Israel, and deprived of its leverage in Washington through oil.

President Reagan keeps preaching. On Tuesday of the past week he said that a ''fundamental objective of our foreign policy'' is a ''resolution of the Palestine problem that satisfies that people's legitimate rights.'' But he can do nothing to promote that ''resolution of the Palestine problem'' without a head-on battle in Congress which traditionally has supported Israel's armed forces and economy.

The record shows that Israel's American supporters can win almost any vote in Congress touching Israel. When asked whether the President would back up his Mideast peace plan by proposing to withhold funds for Israel, a prominent Republican senator replied: ''With an election coming up next year, not a chance.''

This combination of conditions and events has turned the idea of Israeli withdrawal from the Arab territories occupied in 1967 into a will-o'-the-wisp. It still exists in theory as an objective of American policy. It is supported by the international community in the United Nations. But the serious question today is not whether Israel will withdraw from the Arab territories it has occupied for more than 15 years. The serious question is how much more Arab territory it is going to want in addition.

Israeli supporters in Washington are now talking about the ''necessity'' of keeping their troops in southern Lebanon for several years ahead. The ''agreement'' which Israel wants with Lebanon would provide for stationing Israeli ''observer'' teams in southern Lebanon.

Also, Israel is asking that the ''Haddad militia'' force be integrated into the regular Lebanese Army and left where it is in southern Lebanon. The ''Haddad militia'' has been raised, equipped, paid, and trained by Israel.

As of this weekend there does not appear to be any effective barrier in the way of Menachem Begin's expansion policy. Unless something unforeseeable occurs, he will consolidate the territories occupied since 1967 into Israel, secure continuing control over southern Lebanon, and then take a look at Jordan. Does he want some of that land, too?

Among Begin supporters there is talk of Jordan being part of the land of Palestine. And it is argued by some that Israel is the legal successor to the former British mandate over Palestine and is, therefore, entitled to do what it chooses with the whole of Palestine, including the kingdom of Jordan.