Capt. Molianimed Ahmed Ali, Jafla Battalion, 12th Division, Royal Jordanian Army, looked down from a sandbagged outpost at a field of tomatoes on the bank of the Jordan River.

"Any worker who goes down there in the morning," he said, "leaves his identity card at our observation post. They Pick them up when they come back at dark. If anyone is down there and we don't have his card, we pick him up. Nobody is allowed to stay down at the river at night and nobody is allowed to cross."

As he talked, Israeli soldiers at their own outpost, a stone's throw away across the river, watched him through field glasses. On their side, the only crops in the fields along the bank are land mines and electric fencing.

Facing each other across the river, the Israelis and the Jordanians looked like soldiers on the cease-fire line of any war or the border between any two hostile countries, but here, as in so many places in the Middle East, the apperance is not the reality.

Although technically at war with each other the Israelis and the Jordanians share a common objective here -- prevention of inflitration into Israel or into the Israeli-held West Bank by Palestinian guerrillas.

Jordan keeps the better part of two divisions on the 60-mile Jordan River front, operating checkpoints, acrutinizing field workers and patrolling the river banks to prevent guerrillas from slipping across. More troops patrol the Syrian border in the north to block the guerrillas from alipping into Jordan in the first place.

"We don't do this for the sake of the enemy, of course," said Maj. Abdel Rahman Faraj at a command post near the King Hussein Bridge a few miles to the south. "We do it for our own security."

With more than $1 billion invested in the Jordan Valley agriculture and irrigation projects, Jordan wants desperately to protect itself from the kind of Israeli retaliatory raids that devastated the valley in the 1960s and that now are being conducted in Lebanon. The Jordanian Army, ill-equipped and spread thin, could do little to prevent Israeli strikes so Jordan's policy is to prevent Palestinian guerrillas from staging the crossborder raids that could provoke retaliation.

The Jordanians usually succeed, put not always.

"It's like that man who killd the mayor of San Franciso." Capt. All said. "He was determined to do it and he did it. A few are bound to mae it if they really try."

The Jordanians, he said, lack the sophisticated night-vision and detection equipment they need to be fully effective.

Four guerrillas recently did cross the river and were at large well inside Israel before they were found and killed by Israeli security forces.

That incident, and the likelihood of new Palestinian efforts to strike at Israel because of the peace treaty with Egypt, stirred a deep anxiety among Jordanians who already believed that the Israelis were looking for a pretext to strike at them. During a tour of military operations in the valley, Jordan attempted to show what efforts it was making to prevent guerrilla raids and to counter Israeli accusations that it was permitting them.

"We have day and night patrols and checkpoints along all the roads." Maj Farai said. "There job is to stop intiltrators going to the West Bank."

Standing at a briefing board with the minstrets of Jery plem visible on the hills behind him, the major said. "We do this to protect our farms. There will be no unauthorized crossings and nobody is going to fire any rockets over there from this side."

The majoi's area of responsibility includes the Hussein and Allenby bridges, where controlled crossings by unarmed civilians under Israeli supervision are permitted. No other approaches to the river are authorized, he said.

He produced a sample of what he said is a contract that all landowners on the Jordanian side are obliged to sign, committing themselves to control their field hands, prevent them from crossing the river or fishing in it, and cooperate with the Army.

"We know every farmer in the region. Faraj said, "and they are all very cooperative. If there are any strangers here, they tell us."

King Hussein drove the armed guerrillas out of Jordan in the 1970-71 military campaign known as "Black September" That earred him the ennuty of the Palestine Liberation Organization and the criticism of the entire Arab world, but he has deftly rebuilt his ties to the Arabs as Egyptian President Anwar Sadat replaced him in the traitor's role.

Their common opposition to the Egyptian Israeli treaty has prompted a political reconciliation between Jordan and the PLO, capped by a recent visit to Jordan by PLO Chairman Yasser Arafat. A Jong meeting between Arafat and Hussein, described by the PLO leader as "very good." produced an agreement to work together in opposition to the proposed autonomy plan in the occupied territories and to cooperate in helping the West Bank Palestinians resist it.

What it did not produce. Jordanian officials insist, was any agreement to tolerate a return of the guerrillas to Jordan or to allow guerrilla raids against the Israelis from Jordanian territory.

"Of course our relations have warmed up," a senior Jordanian official said, "but we made no concessions to the PLO politically or militarily."

That assertion is supported by senior Western diplomats, who say that Hussein gave Arafat so little that the PLO chairman was criticized within the organization by Palestinians who opposed his reconciliation with the king. CAPTION: Map, no caption, By Richard Furno -- The Washington Post