Saudi Arabia, with 200,000 military and National Guard troops, is the largest oil producer and exporter, with an output of nine million barrels of oil per day, according to Middle East Newsline. The Arab kingdom is the third largest supplier of oil to the United States, with more than 1.55 million barrels per day.

The scenario was outlined by Michael O'Hanlon, a senior fellow of the Brookings Institution, who cited a Saudi coup as one of several threats to the United States.

"How should the United States respond if a coup, presumably fundamentalist in nature, overthrows the royal family in Saudi Arabia?" O'Hanlon asked. "Such a result would raise the specter of major disruption to the oil economy."

The response could include the deployment of three U.S. Army divisions backed by fighter-jets and airborne early-warning and alert aircraft. In all, the U.S.-led mission could include up to 300,000 troops.

Congressional sources said the House hearing, which focused on future threats in the Middle East and other regions, marked increasing U.S. concern of Saudi instability. They said the open hearing echoed a series of briefings on Saudi and Gulf Arab instability given by non-government analysts to the State Department, Defense Department and National Security Council since 2002.

The House committee was told that U.S. concern of a Saudi coup appears greater than ever. O'Hanlon said such a coup would also destabilize Pakistan, a nuclear power since 1998.

"This type of scenario has been discussed for at least two decades and remains of concern today — perhaps even more so — given the surge of terrorist violence in Saudi Arabia in recent years as well as the continued growth and hostile ideology of Al Qaida along with the broader Wahabi movement," O'Hanlon said. In his testimony, O'Hanlon envisioned a Saudi coup as resulting in the emergence of what he termed a fundamentalist regime intent on acquiring nuclear weapons. Another prospect was that the new regime would seek to disrupt the oil market.

"Indeed, it might be feasible not to do anything at first, and hope that the new regime gradually realized the benefits of reintegrating Saudi Arabia at least partially into the global oil economy," O'Hanlon said. "But in the end the United States and other western countries might consider using force."

O'Hanlon envisioned a U.S.-led military operation designed to seize Saudi oil wells, located along the eastern coast. Washington and its allies would place the proceeds from Saudi oil sales into escrow for a future pro-Western government in Riyad.

A U.S.-led military force of 300,000 would be required to secure the entire Saudi Arabia, O'Hanlon said. He said about 10,000 troops could capture eastern Saudi Arabia, which contains virtually all of the kingdom's oil wells. But more than 100,000 additional troops would be required to protect the wells and other vital infrastructure.

"An operation to overthrow the new Saudi regime and gradually stabilize a country of the size in question would probably require in the vicinity of 300,000 troops, using standard sizing criteria," O'Hanlon said. "So in fact a coastal strategy, while easier in some ways and perhaps less bloody in the initial phases, could be fully half as large and might last much longer."