WITH U.S. FORCES, SAUDI ARABIA, DEC. 18 -- The commander of U.S. Marines in Operation Desert Shield said today Iraqi President Saddam Hussein long ago "missed his chance" to invade Saudi Arabia, but he warned that a war against Iraqi forces is likely to move quickly and be "fairly brutal."

"I don't know how many casualties we're going to take, and I've thought a lot about it," Lt. Gen. Walter Boomer said in an interview. "If one side does everything right and the other makes mistakes, then it could be over quickly. On the other hand, the potential is there -- and I stress the word 'potential' -- for significant casualties."

Like other top-level Desert Shield commanders, Boomer refused to discuss specific U.S. tactics should war break out, but he envisioned a large-scale tank battle in the Arabian Desert:

"In general, the war will probably be fast-moving in which all supporting arms will be brought to bear," Boomer said. "It will be fairly brutal because the forces that are aligned against us are powerful."

Boomer currently commands more than 60,000 Marines in the 1st Marine Expeditionary Force, and is currently being reinforced with an additional division expected to raise total Marine strength in the Persian Gulf region to about 90,000 soldiers.

In all, the approximately 480,000 U.S. and allied forces are confronting about 500,000 Iraqi soldiers near the Saudi border in Iraq and occupied Kuwait.

Boomer and the 1st Marine Division arrived in Saudi Arabia in mid-August with Iraqi troops firmly in control of Kuwait and poised on the Saudi border, apparently ready to invade.

"I had a couple of feelings," Boomer said. "One, I was absolutely convinced we'd protect this complex {Marine headquarters}; two, I felt it would be a hell of a battle; three, if they came in force, we'd need help."

"But I didn't envision another Dunkirk," he added. "We had ammunition and supplies, and we manned up very quickly." Still, he said, he didn't feel "confident" that the Marines were out of danger until mid-September. By that time the 1st Division and vanguard Army divisions had built a deterrent force capable of defending against any Iraqi offensive.

"It's hard to ascertain what his {Saddam's} goals are and were politically," Boomer said. "But strictly from a military point of view, yes, he missed his chance, and I think he was caught off guard by the fact that we came in really quickly."

A future offensive by Iraq against Saudi Arabia, he said, is "not possible. He could try, but he'd be very foolish."

Boomer repeated his conviction that the 2nd Marine Division, which began arriving in Saudi Arabia by aircraft last week, would be fully equipped, trained, deployed and ready to fight by Jan. 15. A U.N. Security Council resolution has authorized military action against Iraq if it fails to withdraw its forces from Kuwait by that date.

"We're not armored divisions," he said, "but each of the divisions will have tank battalions." In addition, he said, his troops have a lot of antitank weaponry and aircraft: "What you really have here I would describe as two mechanized divisions."