Iran's stepped-up arming of Shiite militiamen in southern Iraq who are targeting American troops may be designed to trigger a "Beirut-like moment" of mass U.S. casualties, the Obama administration's nominee to be the next chairman of the Joint Chiefs of Staff told Congress on Tuesday.

Army Gen. Martin Dempsey, in testimony before the Senate Armed Services Committee, was asked by Sen. Joseph Lieberman, I-Conn., about a previous statement Dempsey had made in which he expressed concern that Iran might miscalculate the level of U.S. resolve to assist Iraq.

Dempsey said his Iraqi contacts have told him it appears "Iran's activities in southern Iraq are intended to produce some kind of Beirut-like moment and, in so doing, to send a message that they have expelled us from Iraq." He did not specify which Iraqis said this, although he noted that their view is "in some cases supported by intelligence."

Dempsey was alluding to the 1983 bombing of a Marine barracks in Beirut that killed 241 U.S. service members and drove the U.S. out of Lebanon.

In follow-up questioning on this issue, Sen. Lindsey Graham, R-S.C., asked Dempsey what Iran should know about prospects for driving the U.S. out of Iraq by inflicting mass casualties.

"It would be a gross miscalculation to believe that we will simply allow that to occur without taking serious consideration of reacting to that," he replied.

The U.S. currently has about 46,000 troops in Iraq; virtually all of them are due to leave by the end of this year, although senior U.S. officials have said they believe Iraq will need U.S. security assistance beyond 2011. Dempsey said he would favor extending the U.S. troop presence, if Iraq asks.

Dempsey, who currently is the Army's chief of staff, fielded questions from the committee on a wide variety of topics, but the predominant issue was the U.S. debt crisis and the prospects for further cuts to the defense budget. Dempsey said he realizes that if he is confirmed by the full Senate _ as is widely anticipated _ he expects to lead a military that faces "a new fiscal reality."

He said the military needs to contribute to deficit reduction in order to avoid the impression of being isolated from the rest of society.

Dempsey also said he expects cybersecurity to be one of the defining issues of his tenure. And he expressed support for President Barack Obama's decision to withdraw 10,000 troops from Afghanistan by the end of this year and another 23,000 by September 2012.

Obama picked Dempsey to succeed Navy Adm. Mike Mullen as Joint Chiefs chairman. Mullen is due to retire Oct. 1.

Mullen's departure follows the retirement of Defense Secretary Robert Gates last month and the pending move of Gen. David Petraeus from commander of international forces in Afghanistan to director of the CIA. Former CIA chief Leon Panetta has taken over for Gates at the Pentagon.

Next week, Marine Gen. James Cartwright will finish his term as vice chairman of the Joint Chiefs and retire, to be succeeded by Navy Adm. James Winnefeld, who is expected to be confirmed by the Senate shortly. Also awaiting Senate approval is the nomination of Gen. Ray Odierno to succeed Dempsey as Army chief.

The new lineup appears to offer the promise of stability in Obama's relations with the military as the wars in Iraq and Afghanistan wind down. The president will look to Dempsey and Panetta for advice on managing future defense spending cuts without undercutting military strength and morale.

The chairman of the Joint Chiefs is not in the chain of command that runs from the president to the secretary of defense to commanders in the field.

Dempsey has taken an unusually twisted path to the military's top job. He has joked that he may go down in history as the shortest-serving Army chief. He took that job April 11. Barely a month later Obama picked him to succeed Mullen, reflecting a presidential change of heart about Cartwright, who for months had been widely assumed to be a shoo-in for the prestigious post.

After two tours in Iraq _ first as commander of the 1st Armored Division in Baghdad and later as commander of the organization charged with training and equipping Iraqi security forces _ Dempsey was serving behind the scenes as deputy to Adm. William J. Fallon, head of the U.S. Central Command, when Fallon resigned suddenly in 2008. Gates installed Dempsey as interim commander, even though he had already been nominated and confirmed to become the top commander of U.S. Army forces in Europe.

After several months Petraeus took over at Central Command and Dempsey was given command of the Army's Training and Doctrine Command at Fort Monroe, Va., where he developed the Army's thinking on how to prepare for future wars. There, he preached "the gospel of adaptation" _ a conviction that in uncertain times, soldiers and their leaders must be versatile and open to new ways of doing things.

Dempsey, who grew up in New Jersey and New York, received a master's degree in English from Duke University in 1984 and then taught English at West Point. He also earned master's degrees from the Army's Command and General Staff College in 1987 and from the National War College in 1995.

___

Robert Burns can be reached at http://twitter.com/robertburnsAP