In the diplomatic world this week one of the most interesting and important questions is whether President Reagan of the United States is really in earnest about the Middle East peace plan he launched on Sept. 1.
If the President is serious, and is willing to put his full influence behind his own plan, then the chances are excellent that Arabs and Israelis can be led (both sides protesting and dragging their heels, of course) to that ''comprehensive settlement'' that has been the aim of American diplomacy ever since the 1967 Arab-Israeli war.
Those who work on the problem tend to agree that just ahead will probably be both the best chance yet to reach a stable peace between Israelis and Arabs and also, probably, the last chance for such a settlement for years to come.
But this window of opportunity must be opened and used promptly. Otherwise Israeli Prime Minister Menachem Begin will have planted so many Israeli colonists in the occupied territories, and squeezed out so many of the Arabs, that the Arab countries, moderates as well as radicals, will conclude that war is their only recourse.
If the time comes when the Arabs lose confidence in Washington's seriousness about the Reagan plan, they presumably will invite the Soviets back into the Middle East; Egypt presumably will cancel its peace treaty with Israel; and everyone will settle down to prepare for the next war. Such a war would, of course, interfere with the flow of oil from Saudi Arabia to the West and carry with it the risk of another Soviet-US confrontation.
Little that is visible has been done in the two months between the announcement of the Reagan plan and now. But this is neither surprising nor damaging to peace prospects.
It would have been unrealistic to expect the President to devote much time to the Middle East on the eve of the mid-term elections. It would have been even more unrealistic to expect him to take a strong stand against Prime Minister Begin's policies and purposes at such a time.
In fact, the time has not been wasted and a good deal of stage setting has taken place.
When the Reagan plan was launched Sept. 1, the Begin government in Israel immediately denounced it. The members of the Arab League met in Fez, Morocco, from Sept. 7 to 9. They agreed upon a plan of their own which differed from the Reagan plan, but contained a vague but implicit acceptance of the existence of Israel. Given a real interest in peace on all sides, the two plans could conceivably be reconciled.
The Arabs have had time since then to mull over the main features of the two plans and possibilities for reconciliation. An Arab League delegation led by King Hassan II of Morocco has called on the President in Washington. PLO leader Yasser Arafat has been to Amman to talk with King Hussein of Jordan.
This week Mr. Arafat was quoted by the PLO news agency as saying that he was ready to establish a ''confederation '' between Jordan and an independent Palestinian state after the latter had been formed. Still unclear is who will speak for whom. Mr. Arafat says the King Hussein will not speak for the Palestinians, whose sole voice remains the PLO.
But clearly the two men are trying to work out a way to respond to the American proposals - provided they are first convinced that President Reagan is in earnest. The essential fact they face at this stage is that Mr. Arafat would be repudiated by the radicals in the PLO movement and King Hussein would be regarded by his own people and by other Arab leaders as a traitor if the two men let themselves get into negotiations that did not guarantee the liberation of the occupied territories from Israeli rule.
Is Israel going to be allowed to hang onto the West Bank and Gaza, probably prompting many of the Arab inhabitants of those territories to leave?
Or is Mr. Begin going to be persuaded or forced to let the bulk of those territories revert to Arab self-rule as contemplated by United Nations Security Council Resolution 242 and by the Camp David agreements?
Only one person can decide which course lies ahead. Only the President of the United States has the power and the influence.
If he is ready and able to use it, then a peaceful settlement is conceivable. If he is not prepared or able to use it, King Hussein and Mr. Arafat dare not get themselves caught in the inevitable failure of the negotiations.
On Tuesday, the American mid-term elections will be out of the way. Mr. Reagan will have at least six months, perhaps nearly a year, during which he could devote substantial time and effort to the Middle East before he will be overwhelmed by planning for the 1984 presidential elections. The first and third years of a presidency are the best times for doing difficult and unpleasant tasks. Years two and four are too dominated by politics to permit orderly government.
This weekend US Ambassador Morris Draper was scheduled to return to Beirut in charge of negotiations for the withdrawal of both Israeli and Syrian forces from Lebanon. Ambassador Philip Habib is at work in Washington on the larger framework for peace.
If these able American diplomats can persuade King Hussein and Mr. Arafat that Reagan will ''stay the course'' with his Sept. 1 peace plan, then serious negotiations may begin. The stage is set. The curtain can go up as soon as the Nov. 2 elections are over.
But will the President in Washington play the role his ambassadors have written for him? If you can answer that question, you can know whether there is peace, or more war, ahead in the Middle East.