Shortly after he was promoted to four stars, Gen. Martin E. Dempsey made a decision that spoke volumes about how he views the future of war.

The Army had spent hundreds of millions of dollars over the previous decade on an annual war game designed to peer 15 to 20 years into the future and envision what kinds of technology and fighting concepts the military would need to prevail in the next major war. The game involved dozens of role players and an army of defense contractors waging computer-simulated battles.

Dempsey, whom President Obama will nominate on Monday to be chairman of the Joint chiefs of Staff, killed the war game in 2009. He replaced it the next year with a series of seminars devoted to producing more flexible and free-thinking officers at all levels.

“Marty believed a good leader could succeed and rise above concepts [of war] and military doctrine,” said retired Maj. Gen. David Fastabend, who served with Dempsey in Iraq. “For him it is all about leadership.”

Dempsey was sworn in as Army chief of staff only last month, but will give up that position in the fall when Adm. Mike Mullen, the current chairman of the Joint Chiefs of Staff retires. Gen. Ray Odierno, who commanded troops in Iraq from 2008 to 2010, is expected to take over as Army chief. Both positions require Senate confirmation.

In choosing Dempsey for the military’s top job, Obama has selected an officer who in many ways is the polar opposite of Gen. James E. Cartwright, a favorite of the president who was long presumed to be the front-runner for the position. Obama decided on Dempsey only in the past two weeks.

Cartwright, a Marine fighter pilot, is known inside the Pentagon as a tech-savvy introvert who has spent much of the last decade working to ensure that the military is prepared for the next big war. He’s made himself into an expert in cyber and nuclear warfare.

Dempsey has spent much of the last decade leading troops in a messy, low-tech war in Iraq and is deeply skeptical of technology’s ability to alter the basic nature of combat.

“We operate where our enemies, indigenous populations, culture, politics, and religion intersect and where the fog and friction of war persists,” he wrote recently in the introduction to the Army’s main operating concept.

Critics complain that Dempsey has not pushed the Army to think hard enough about how future wars might differ from Iraq and Afghanistan. His outlook, however, reflects the dominant thinking within the Army’s officer corps and the viewpoint of Defense Secretary Robert M. Gates, who has complained that the Pentagon tends to focus on preparing for the next war, a condition he termed “nexwaritis.”

Among his fellow generals, Dempsey is well respected and known as an extrovert, prone to grabbing the microphone and belting out Frank Sinatra standards at formal Army social events.

Dempsey’s convivial nature and close relationships with fellow generals will be tested as the Pentagon confronts a future that likely includes big defense spending cuts and a contentious debate over how best to end the war in Afghanistan.

Like many senior Army officers, Dempsey’s view of combat has been shaped indelibly by his searing experience in Iraq.

In May 2003, Dempsey assumed command of the Army’s 1st Armored Division, which was struggling to put down a growing anti-American insurgency that was taking root in Baghdad.

As the commander of 20,000 soldiers in Baghdad, he oversaw a series of major operations designed to weaken the growing insurgency and buy time for the faltering U.S. reconstruction effort. Some of the big offensives knocked the fledgling insurgency back on its heels, said retired Col. Peter R. Mansoor, who commanded under Dempsey and now teaches military history at Ohio State University.

Other efforts, such as a Dempsey-led operation to target black-market fuel profiteers in the Iraqi capital, were of little worth, Mansoor wrote in “Baghdad at Sunrise,” an account of his brigade’s year at war. The operation, dubbed “Iron Justice,” was the equivalent of “sweeping back the ocean with a broom,” Mansoor wrote.

In spring 2004, as Dempsey’s troops were preparing to leave the country, militia fighters loyal to radical Shiite cleric Moqtada al-Sadr launched a major uprising in southern Iraq and Baghdad.

Dempsey flew to Kuwait, where his troops were preparing to board planes home, to tell them personally that the Pentagon had extended their tour for another 90 days. His forces put down the uprisings, then launched a major effort to convince the militia fighters to lay down their arms and take jobs cleaning up the battle damage.

“He learned a lot in Baghdad and was able to apply it when it mattered most in the counteroffensive against Sadr’s forces,” Mansoor said.

Beginning in August 2005, Dempsey spent two years overseeing the development of Iraqi army and police forces that were supposed to take over the battle against insurgents. He had inherited a poorly led force that was being torn apart by Iraq’s ethnic and sectarian tensions.

Dempsey led a politically sensitive effort to purge weak and corrupt leaders from the Iraqi military, including many who had close ties to the country’s senior politicians.

“We went through quite a drill trying to get the Iraqi forces to take responsibility and deploy,” said Fastabend, who worked closely with Dempsey in Iraq. “Marty gave very honest assessments of what they could do and what was likely to happen if we pushed too fast.”

Dempsey has spent most of the last two years in charge of the command that oversees Army training and the doctrine that guides how it fights. His tenure emphasized that even the most sophisticated technology couldn’t erase uncertainty on the battlefield or substitute for the judgment of a soldier on the ground, a lesson the Army learned through painful experience in Iraq and Afghanistan.

“His message was that we can’tput ourself in a hole now because we are so worried about what is going to happen 25 years from now,” said one officer who worked closely with him and was granted anonymity to talk about a superior.

Some of Dempsey’s critics charged that he was too focused on the wars in Iraq and Afghanistan and had not pressed the Army hard enough to think about future battles against higher-tech foes.

“Marty reflected the broader Army consensus that we have to focus on the near term more than future warfare requirements,” Fastabend said. “In that sense, he was representative of just about everyone in the Army.”