Some top civilian aides to Rumsfeld had argued that the Iraqi Army could be defeated and Saddam Hussein ousted by a smaller number of troops relying on speed, surprise, air power, psychological operations, and help from Iraqi opposition groups, the officials and analysts said.

But they said that General Tommy Franks - the commander of American forces in the Middle East as head of the Tampa-based US Central Command - and other top officers countered that Iraq is not Afghanistan and its forces could be much more formidable than the ragtag Taliban militia.

Franks and others insisted on an invasion force big enough to deal with a worst-case scenario in which Iraqi resistance does not quickly collapse, as widely forecast, and US troops become embroiled in heavy combat in Baghdad or other populous areas.

''Franks wanted to go in there loaded for bear,'' said one senior military official, who, like others, spoke on condition of anonymity. Rumsfeld's ''approach was you need to justify what you have.''

A ground force of around 130,000 American troops would be a fraction of the half-million-strong contingent that drove the Iraqi Army out of Kuwait in the 1991 Persian Gulf War.

But the 375,000-man Iraqi Army is plagued by low morale and obsolete weapons. Those factors plus new precision-guided bombs, other advanced technologies and weapons, better tactics, and the experience gained in wars in the Balkans and Afghanistan give the US military a commanding edge, experts said.

Barry McCaffrey, who commanded the 24th Infantry Division during the 1991 conflict, said with the right force, the right options, and an excellent psychological operations campaign, the US will have ''a relatively short, sharp attack'' that will bring down the Iraqi regime.

Rumsfeld has declined to discuss any details of a possible Iraq invasion. He repeatedly has said that President Bush has not yet decided whether to make good on his threat of military attack if Hussein refuses to comply with a new round of United Nations weapons inspections.

Before accepting Franks's argument for a large ground force, Rumsfeld repeatedly challenged the general to explain his reasoning, said senior defense officials.

''The secretary accepts the fact that you have to plan for a worst case,'' said a second senior defense official. ''There are others who have agendas and who surround the secretary and who challenge from an unintelligent point of view. Sometimes it's pure stupidity. But the secretary is very, very practical.''

In fact, it is not clear that Rumsfeld ever supported his aides' push for an invasion force of 80,000 troops or less. The second senior defense official said news reports of those options were ''disinformation.''