When the United States joined Arabs and other members of the United Nations in condemning the Israeli killing of 21 Palestinian rioters and wounding 140 others in Jerusalem this week, it did not mean that America was suddenly abandoning its historic support of Israel.

The traditional U.S. supply of money and weapons to Israel will surely continue, but the massacre - aside from its shocking, unnecessary taking of life - threatens to upset existing Arab support in the Persian Gulf crisis and give Iraq's Saddam Hussein a propaganda weapon.While Saddam has received loud support from many Palestinians, Israel had been asked to keep a low profile in the Gulf crisis while Washington tried to bolster an anti-Iraq coalition among Arab states. The killings this week have damaged that effort and taken Middle East attention off Iraq and focused it directly on Israel.

Under the circumstances, the United States had to react quickly to limit damage from the shootings and keep the Israeli-Palestinian question from being dragged into the Iraq-Kuwait issue any more than it has. As a result, President Bush immediately criticized Israel for its use of "excessive" force against the stone-throwing rioters, and a U.S.-sponsored resolution in the United Nations calls for an investigation into the matter.

There is plenty of blame to go around. The riot began when more than 3,000 Palestinians atop the Temple Mount began bombarding with stones the peaceful Jewish worshipers at the historic Wailing Wall below. The 40 Israeli police on hand were quickly overwhelmed and resorted to gunfire.

Clearly, the police were too quick on the trigger, despite the violent situation. That can be seen from the casualty figures; a handful of Israelis were hurt, none seriously, while 19 Palestinians were killed, more than 100 wounded. Later rioting in reaction to the shootings added more casualties. It also seems clear that the Palestinian attack was not spontaneous. It was planned in advance and rocks were stockpiled.

Yet the question is not who started it. The more important issue is how to keep Iraq from changing the subject. Israeli-Palestinian problems are important, but right now, the more vital issue has to do with the conquest of Kuwait and the threat to the world's oil supplies.

The best thing Israel can do for itself and its friends at the moment is to be as helpful, remorseful and quiet as possible. The worst thing would be for Israel to be defiant, uncooperative and arrogant; that would only undermine the fragile U.S.-Arab alliance and serve the interests of Iraq.

The United States has Saddam as a threat in front, the Arabs as highly sensitive and uneasy allies to the sides, and Israel, a long-standing client and ally at the rear. Right now, it's hard to say who is causing the most trouble for the crucial American balancing act.