In the 1980s, the U.S. military was in the middle of a transformation. The Vietnam War was over, and a force once staffed with drafted troops who had fought and died in the jungles of Southeast Asia was transitioning to ranks filled solely with volunteers.

In Nebraska, Tim Walz was one of those volunteers.

Mr. Walz, now Minnesota governor and the presumptive Democratic candidate for vice president, raised his hand to join the Army National Guard just two days past his 17th birthday on April 8, 1981. In a career in the military that spanned three decades, he battled floods, managed an artillery unit and achieved one of the highest enlisted ranks in the Army. He also navigated a full-time job teaching social studies alongside his part-time military occupation as an enlisted combat arms soldier, a role that trained him for war.

Mr. Walz never went to war. Most of his service covered a period when America was bruised from foreign entanglements and wary of sending troops into combat overseas for long stretches. And it ended when Mr. Walz was 41, as the military ramped up for war after Sept. 11.

Since being picked as Vice President Kamala Harris’s running mate this week, he has found himself facing allegations previously aired by Minnesota Republicans and newly amplified by JD Vance, former President Donald J. Trump’s running mate.

Those criticisms center on Mr. Walz’s decision to retire from the Army in 2005, the year before his artillery battalion deployed to Iraq. He was thinking seriously about a run for Congress and spoke with other soldiers about being torn between his loyalty to his fellow troops and his desire to move on with his life. At the time, there were vague expectations that the unit might deploy, but actual orders came several months later.

The unit deployed to Iraq for more than one year beginning in 2006. During that time, soldiers in the unit provided security for transportation convoys and other tasks common in a combat zone.

Supporters say he served honorably for 24 years and had long since earned the right to retire from the Guard when he chose.

“In talking with him, it was a hard decision,” said Joseph Eustice, a soldier who served under Mr. Walz until his departure. “The guy I knew in the Guard, I’d walk through a wall for.”

But Thomas Behrends, a retired command sergeant major in his Guard unit, took Mr. Walz’s place for the Iraq deployment and said Mr. Walz was a “sell out” to his unit. “Whatever reason he had, he’s never explained it,” said Mr. Behrends, who wrote an editorial in 2018 speaking out against Mr. Walz during his campaign for governor.

In 2006, when Mr. Walz first ran for Congress, one critic who served in the Minnesota National Guard claimed that he was misleading people into thinking his record included a combat deployment. Mr. Walz pushed back strongly.

“There’s a code of honor among those who’ve served, and normally this type of partisan political attack comes only from one who’s never worn a uniform,” Mr. Walz wrote that year in a letter to the editor of The Winona Daily News, a newspaper in the district.

Even his most vocal opponents in the Guard agree that Mr. Walz was a respected soldier — someone who did his job dependably and could be counted on to take care of his troops.

Mr. Walz was drawn to the military the same way many Americans are: It was a family business. His father was a veteran, and his older sister had joined the military three years before him. His father died of lung cancer soon after Mr. Walz enlisted.

“I think it was instilled in us. It was just something that I think was more of an expectation,” Mr. Walz said in a 2009 Library of Congress interview. “I never really thought of a time when I wouldn’t join some branch.”

On the day he enlisted, the 17-year-old Walz was picked up after a high school track practice. After going to a nearby town, passing cornfields and flat roads, he met a lieutenant in charge of the local Guard unit. From there, he said in the 2009 interview, his 24 years of military service began.

Mr. Walz’s first job in the Nebraska Guard was in the infantry. In the 1980s, that meant Private Walz quickly learned how to fire an M16 and shoot, move and communicate in small units as he completed his training between junior and senior year of high school at Fort Benning in Georgia.

With the threat of war with the Soviet Union still present, Army recruiting commercials highlighted a modernizing American military: Newly formed Ranger units descended from Black Hawk helicopters and Apache gunships buzzed the ground, all with the Army’s famous motto “Be all you can be” blaring in the background.

As Mr. Walz climbed through the lower enlisted ranks, he bounced around the Midwest and the South, taking jobs in Texas and Arkansas before settling on his career path as a teacher. He figured that teaching left ample room in his schedule to train with the Guard. Those requirements frequently called on those in the Guard to show up for one weekend a month and a few weeks a year (sometimes in the summer months and sometimes for longer stretches of time).

“For many of us in the National Guard, there was an awful lot of teachers,” Mr. Walz said in the Library of Congress interview. “It was a very compatible profession. It felt like there was a lot of crossover and skill sets that worked there.”

Throughout the 1990s Mr. Walz performed the standard duties called upon by National Guard units — flood cleanup and recoveries from forest fires and tornadoes. In 1996 he changed jobs from the infantry to artillery and joined 1st Battalion, 125th Field Artillery in the Minnesota National Guard after he married and moved to Mankato, Minn. A year later he was in Norway on a training exercise.

“He was a good soldier,” said Bill Kautt who was Mr. Walz’s battalion commander during the short deployment to Norway. “I think he was a respected member of the Guard and he did his job.”

Mr. Walz was the chief of one of the unit’s howitzer batteries and joined the exercise as the battalion’s operations sergeant in the command group and worked closely with Mr. Kautt. In their downtime, he recalled connecting with Sergeant Walz as both were social studies teachers at the time. And, they both coached school athletic teams.

“I was very impressed with him,” Mr. Kautt said. “I would consider him energetic. He was dependable and he was willing to do whatever we asked him to do.”

But his time in an artillery unit came at a cost to his health, despite not seeing combat. The deafening booms and shock waves from howitzer barrels and their projectiles, which sound like freight trains as they fly overhead, left Mr. Walz with hearing loss and tinnitus in both ears, according to a 2018 report from Minnesota Public Radio. In 2005 he underwent corrective surgery to alleviate the problem, the report said.

As Mr. Walz navigated the duties, training and health issues common in any military unit, especially those in combat arms like field artillery, he managed to keep his dual roles — soldier and teacher — separate.

“He was a coach and a teacher first,” said Noah Hobbs, a student of Mr. Walz in 2005 at Mankato West High School. He added that Mr. Walz did not boast about his military service, nor bring it up frequently in the classroom.

But the fact that Mr. Walz was in the military was common knowledge among the student body.

Mr. Walz’s relatively peaceful time in uniform — having never been deployed as part of operations in Grenada in 1983, Panama in 1989 or the Gulf War in 1990 — changed on Sept. 11, 2001, when the terrorist group Al Qaeda killed nearly 3,000 people in New York, the Washington, D.C., area and Pennsylvania during a brazen strike on American soil.

Emily Falenczykowski-Scott, 41, saw the attack unfold on a television in the lobby of Mankato West High School and remembers Mr. Walz running by. He stopped when he saw her. “Emily, thousands of people have already died. This is war,” she recalled him saying.

In the twilight of his time in the Guard, before he retired in 2005 to run for Congress, Mr. Walz deployed to Vicenza, Italy in 2003. His artillery battalion was charged with guarding entry control points at Air Force bases that were supporting the war in Afghanistan as U.S. forces sought to capture or kill Al Qaeda’s leadership there.

After that deployment ended in 2004, Mr. Walz made the decision to leave the Guard as he considered the run for Congress and as rumors swirled about a potential deployment to Iraq. He had been promoted to command sergeant major — a rank few soldiers reach.

But in the end, he retired as a master sergeant because he never completed the coursework — which would have taken 664 hours online and 86 hours in person to complete — required to keep the rank of sergeant major.

Republicans have accused Mr. Walz of inflating his credentials, reviving a dispute from his past political campaigns over which designations Mr. Walz should be entitled to use as his exit rank.

On the Harris campaign site, he is described as a “master sergeant.” But on his governor’s page bio, he has kept the mention of “Command Sergeant Major.”