ASHINGTON, July 31 In the first public hearings on the administration's goal of ousting Saddam Hussein from the Iraqi presidency, an array of experts warned a Senate committee today that an invasion of Iraq would carry significant risks ranging from more terrorist attacks against American targets to higher oil prices.
Opening a Congressional debate that is almost certain to gain momentum into the fall, the witnesses including former senior military officers and nongovernment experts on Iraq expressed confidence that American forces would prevail in an invasion. But they said it would be a difficult fight, requiring a major commitment of troops and the support of many allies.
They also offered sharply different prescriptions for dealing with Mr. Hussein, with some calling for a swift, large-scale attack to depose him before his biological and nuclear weapons capabilities mature and others arguing instead for a tighter embargo and more aggressive weapons inspections to contain him.
But there was a broad consensus among the varied experts that if President Bush decided to use military force to remove Mr. Hussein as many in Congress expect the Pentagon could not assume that the Iraqi military would collapse without a fight or that Iraqi opposition forces could carry on the fight alone.
Rather, the experts said, the military would need to deploy tens of thousands of ground troops as well as many aircraft, ships and armored vehicles to ensure victory. And the administration should plan on keeping forces in Iraq for many years to help rebuild it, the experts added.
"I think it is incredibly dangerous to be dismissive" of the Iraqi military, said Anthony H. Cordesman, a former Pentagon official who is now a senior fellow and Iraq expert at the Center for Strategic and International Studies, a nonpartisan policy organization in Washington. "To be careless about this war, to me, would be a disaster."
The experts also agreed that they consider Mr. Hussein a major threat to world peace because of his aggressive efforts to obtain biological and nuclear weapons. But estimates of when he might actually develop those weapons ranged from a few months to several years.
The hearings before the Senate Foreign Relations Committee, which will continue on Thursday, are taking place at a time of growing concern in Congress that the administration is moving rapidly, and without public debate, toward a full-scale military assault on Iraq as part of its stated goal of "regime change" in Baghdad.
"This is not an action that can be sprung on the American people," said Senator Richard G. Lugar, Republican of Indiana, in an opening statement. "Public debate over policy is important to the construction of strong public support for actions that will require great sacrifices from the American people."
There is also an emerging debate among legislators over whether Mr. Bush would need Congressional approval for an invasion of Iraq. Some administration officials have said they do not need approval, but the White House has not taken an official position on the issue.
Senator Trent Lott, the Republican minority leader, told reporters today that he did not think the administration needed Congressional approval for a major assault. He said that authority had been granted last fall in a resolution supporting military action against Al Qaeda.
"I suspect that Al Qaeda elements are in Iraq," Mr. Lott said. "The resolution we passed, we made it very clear the president has the authority to pursue the Al Qaeda wherever they may be found, in whatever country, which could very well include Iraq."
But Senator Thomas A. Daschle, the majority leader, said he knew of no intelligence confirming Mr. Lott's assertion. He argued that Congressional support for an invasion would help the administration build broader support for its Iraq policies.
"I think it would be a big mistake for the administration to act without Congress and without its involvement," Mr. Daschle said at the Capitol. "I think there has to be a debate, there has to be some good discussion, there has to be some opportunity for the people to be heard."
The administration chose not to send any representatives to the hearings today, asserting that it had not finalized its plans for Iraq. But Senator Joseph R. Biden Jr., the Delaware Democrat who is chairman of the Foreign Relations Committee, said the White House had indicated it might be ready to participate in hearings in September.
Joseph P. Hoar, a retired Marine Corps general who was commander of American forces in the Persian Gulf after the 1991 war, was particularly skeptical of an invasion, calling it "risky" and perhaps unnecessary. General Hoar also said it was far from certain that Turkey and other allies in the region would allow the United States to use their bases.
But Thomas G. McInerney, a retired Air Force lieutenant general, was more optimistic, arguing that "the most massive precision air campaign in history" could quickly crush Iraq's military and spark an uprising against Mr. Hussein.
The two generals did agree that the Pentagon could not expect Iraqi opposition groups to mount an effective campaign against Mr. Hussein's forces without large numbers of American ground forces to help.
Morton H. Halperin, a senior fellow with the Council on Foreign Relations, echoed concerns in Europe and the Middle East that the United States should use its influence to resolve the Israeli-Palestinian conflict before attacking Iraq. "Especially if there is no progress on the Palestinian issue, it is likely that an American military conquest of Iraq will lead many more people in the Arab and Muslim world to choose the path of terror," he said.