Mattis, who retired in 2013 as a general after a 41-year career in the Marine Corps, cited several conflicts the US has plunged into since the end of World War II to illustrate the consequences of failing to plan for what political conditions war is meant to effect.
"Obviously the U.S. has had trouble in major conflicts in recent years particularly in Afghanistan and Iraq, how can the government handle conflicts differently so interventions can be of greater success?" Teddy Fischer, writing for the Mercer Island High School Islander, asked the defense chief.
Secretary of Defense James Mattis
Secretary of Defense James Mattis
Retired Marine Corps Gen. James Mattis walks out after a meeting with President-elect Donald Trump at the Trump National Golf Club Bedminster clubhouse at Trump National Golf Club Bedminster in Bedminster Township, N.J. on Saturday, Nov. 19, 2016.
(Photo by Jabin Botsford/The Washington Post via Getty Images)
U.S. President-elect Donald Trump and Vice President-elect Mike Pence greet retired Marine General James Mattis for a meeting at the main clubhouse at Trump National Golf Club in Bedminster, New Jersey, U.S., November 19, 2016.
Retired Marine General James Mattis departs as U.S. President-elect Donald Trump walks back into the main clubhouse following their meeting at Trump National Golf Club in Bedminster following their meeting at the main clubhouse at Trump National Golf Club in Bedminster, New Jersey, U.S., November 19, 2016.
U.S. President-elect Donald Trump stands with retired Marine Gen. James Mattis following their meeting at the main clubhouse at Trump National Golf Club in Bedminster, New Jersey, U.S., November 19, 2016.
Retired Marine Corps Gen. James 'Jim' Mattis and Operation Gratitude Founder Carolyn Blashek speak during the DIRECTV and Operation Gratitude day of service at the fifth annual DIRECTV Dealer Revolution Conference at Caesars Palace on July 23, 2015 in Las Vegas, Nevada.
(Photo by Bryan Steffy/Getty Images for DIRECTV)
Egyptian Chief of Staff of the Armed Forces Sami Anan shakes hands with US Commander of the Central Command James Mattis during a meeting in Cairo on March 29, 2011.
(KHALED DESOUKI/AFP/Getty Images)
Retired Marine Corps Gen. James 'Jim' Mattis speaks during the DIRECTV and Operation Gratitude day of service at the fifth annual DIRECTV Dealer Revolution Conference at Caesars Palace on July 23, 2015 in Las Vegas, Nevada.
(Photo by Bryan Steffy/Getty Images for DIRECTV)
Retired Marine Corps Gen. James 'Jim' Mattis, former commander of the U.S. Central Command testifies before the House (Select) Intelligence Committee on 'Threats Posed by ISIL (Islamic State of Iraq and the Levant), AQ (al Qaeda), and Other Islamic Extremists' on Capitol Hill in Washington D.C., September 18, 2014. Yesterday the House approved President Obama's plan to train Syrian rebels to counter ISIL.
(Photo by Allison Shelley/Getty Images)
Marine Corps General James Mattis, commander of the US Central Command, appears before the Senate Armed Services Committee on Capitol Hill in Washington DC, March 1, 2011. Enforcing a no-fly zone over Libya would first require a military operation to destroy the north African nation's air defense systems, top US commander General James Mattis warned Tuesday. A no-fly zone would require removing 'the air defense capability first,' Mattis told a Senate hearing. 'It would be a military operation,' he added.
(CHRIS KLEPONIS/AFP/Getty Images)
U.S. Joint Forces Command Commander James Mattis speaks during the 2010 Atlantic Council awards dinner at the Ritz Carlton Hotel on April 28, 2010 in Washington, DC.
(Photo by Kris Connor/Getty Images)
Kuwait Major General James Mattis, a high ranking Marine commander who also led troops into Afghanistan, visits Living Support Area one in Kuwait near the Iraqi border where troops are poised to begin a war against Iraq if called to do so by the President of the United States.
(Photo by Rick Loomis/Los Angeles Times via Getty Images)
U.S. Secretary of Defence Jim Mattis addresses a news conference during a NATO defence ministers meeting at the Alliance headquarters in Brussels, Belgium, February 15, 2018. REUTERS/Francois Lenoir
NATO Secretary-General Jens Stoltenberg and U.S. Secretary of Defence Jim Mattis attend a NATO defence ministers meeting at the Alliance headquarters in Brussels, Belgium, February 15, 2018. REUTERS/Francois Lenoir
U.S. Defense Secretary Jim Mattis testifies to the House Armed Services Committee on "The National Defense Strategy and the Nuclear Posture Review" on Capitol Hill in Washington, U.S., February 6, 2018. REUTERS/Joshua Roberts
U.S. Secretary of Defense Jim Mattis reviews the guard of honour during a welcoming ceremony in Hanoi, Vietnam January 25, 2018. REUTERS/Kham
U.S. Secretary for Defense, Jim Mattis, sits opposite Britain's Secretary of State for Defence, Gavin Williamson, before a meeting at the Ministry of Defence (MoD) in central London, Britain November 10, 2017. REUTERS/Simon Dawson
U.S. Defense Secretary Jim Mattis waits for the arrival of Turkish Defense Minister Nurettin Canikli prior to a meeting on the sidelines of a NATO defense ministers meeting at NATO headquarters in Brussels, Belgium November 8, 2017. Reuters/Virginia Mayo/Pool
U.S. President Donald Trump (R) and Defense Secretary James Mattis participates in a briefing with senior military leaders at the White House in Washington, U.S., October 5, 2017. REUTERS/Yuri Gripas
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"If you look at the wars from probably Korea, Vietnam, Iraq, dare I say Afghanistan, every time we go into a war and we don't figure out what the political end state is, we get into wars and we don't know how to end them. Then you've got a real problem." Mattis replied, referencing a 2013 Atlantic article by James Wright, a retired Marine and professor at Dartmouth, titled "What we learned from the Korean War."
"The most important thing is, if you have to go to war, then do everything you can not to go to war if at all possible," the defense secretary added. "Then you've got to get the political end state right or you'll never figure out how to end it successfully."
US Marine Corps
Mattis cited the 1991 Desert Storm campaign against Iraq as an exception to the US's half-century pattern of entering conflicts without a planned political end state. In that conflict, Mattis says, President George H.W. Bush formed a coalition and pushed Iraqi forces out of Kuwait, stopping short of invading Iraq, despite calls to do so.
"We went in with more troops than we needed and we ended it quickly, because he had the political end state right," Mattis said. (Some have argued that, despite the limited nature of Desert Storm itself, the sanctions, no-fly zones, and airstrikes on Iraq after Desert Storm form part of a longer campaign of hostilities toward the country.)
The US and its allies are still dealing with the reverberations from the invasions of Afghanistan and Iraq in 2001 and 2003, respectively.
"We are not winning in Afghanistan right now. And we will correct this as soon as possible," Mattis told the Senate Armed Services Committee on in June.
The terrorist group ISIS sprung from the vacuum that emerged in Iraq in the years after the 2003 invasion, in part because the Iraqi military was disbanded.
Iraqi forces and their partners in the US-led coalition recently retook the group's last stronghold in Mosul — giving a boost to the US-led "partnered forces" strategy — but ISIS elements are likely to remain in the country for some time.
The US also faces a challenge with the ongoing conflict in Syria.
Trump has delegated a significant amount of responsibility to US military leaders, including tasking Mattis with determining troop levels for Iraq and Afghanistan. (Though there appear to be limits for Afghanistan.)
Reaching stability in both countries is likely to dominate Mattis' time at the Pentagon.
Elaborating on the example of the first Iraq war, Mattis underlined the importance of aligning domestic constituencies and international partners around a definite, achievable political end state for potential conflicts.
"The short answer is, get the political end state right and then give it the full effort and explain to the American people and the American congress what you're doing and get the whole world behind you," Mattis said. "That shows what happens when you get the political end state right, unlike Korea, Vietnam, Iraq that sort of thing."