Yasser Arafat, the bristly Palestine Liberation Organization chieftain, has been given an ultimatum he can't refuse. He has been told to make up with his archenemy, Syrian President Hafez Assad, and he's doing it.

What makes the reapprochement all the more bizarre is the source of the pressure on Arafat: Iraqi President Saddam Hussein, with the formidable backing of the Soviet Union. Hussein himself is a longstanding enemy of Assad, yet he's the one who ordered Arafat to get along with the Syrian dictator.

It was Assad-supported Palestinians who split the PLO four years ago and tried to assassinate Arafat in the process. But Arafat will cozy up to the hated Assad, in deference to Iraq and to the Soviets, who are too powerful to turn down.

The Soviet strategy, according to intelligence reports, is to create a united, pro-Soviet, Arab front against Israel.

At Soviet prompting, the Palestine National Council, the PLO's "parliament in exile," met April 20-26 in Algeria and achieved stunning progress toward unity. At the previous meeting more than two years ago, Syrian-supported radical factions boycotted the council.

This time they not only showed up, but also reunited with Arafat's more elements. There were occasions when the rapprochement seemed doomed, but each time the Soviet ambassador to Algeria, Vasily Taratuta, secretly intervened to pacify the squabbling Palestinians.

Arafat thought things were going swimmingly until he got word from the Iraqi leader to "go easy on the Syrians" at the conference. "Arafat hit the roof at that," an intelligence source told us. "He was appalled."

Arafat shouldn't have been surprised. His own intelligence sources, probably the Saudis, had warned him that Iraq and Syria were on the verge of patching up their long and bitter hostility.

After a brief display of pique, Arafat went along with Hussein's order. Declarations by the conference that would have antagonized j Syria were dropped.

While the PLO meeting was going on, Assad flew to Moscow on April 23 for a two-day visit, his first in two years. In the backrooms of Moscow, our sources understand, the Soviets leaned heavily on the Syrian leader to talk things out with Iraqi President Hussein.

Just as Arafat had to pay attention to his patron, Hussein, Assad had to heed the advice of his Soviet benefactors. Syria owes the Kremlin more than $11 billion for weapons supplied over the years, and doesn't have the money to pay up. Assad's trip to Moscow was ostensibly to discuss rescheduling this huge debt, and the Soviets used it as leverage to move Syria closer to Iraq.

The Soviet pressure appears to have been decisive. The day after his return from Moscow, April 26, Assad secretly flew off to the Royal Jordanian Air Force bse at Al Jafr for a dramatic meeting with Iraq's Hussein. The two neighbors resolved several issues outstanding between their countries. But the most important difference, Syria's support of Iran in its war against Iraq, remained unsettled at the end of the meeting.