JOINT BASE SAN ANTONIO – FORT SAM HOUSTON, Tex. — Army prosecutors began laying out their case Thursday against the soldier at the heart of the most closely watched military desertion case in decades, arguing that Sgt. Bowe Bergdahl sneaked away from his base in Afghanistan under cover of darkness in an effort to get the attention of his commanding general.

Bergdahl, 29, was charged in March with desertion and misbehavior for allegedly walking off his patrol base alone and without a firearm in Afghanistan on June 30, 2009. He was captured within days by insurgents affiliated with the Taliban, and held until the White House agreed last year to a controversial deal in which he was exchanged for five Taliban detainees held at the military prison at Guantanamo Bay, Cuba. The released Afghans were sent to Qatar.

Bergdahl, who is assigned to the installation here, walked into a small hearing room Thursday morning in a crisp, blue dress service uniform. He responded affirmatively in a soft, deep voice when asked if he understood his right to remain silent.

“Yes sir, I do,” he said, seated at a table while flanked by Army defense lawyers and his civilian counsel, Eugene Fidell.

Bergdahl faces up to life in prison if convicted. He sat ram-rod straight for most of the proceedings, occasionally writing in a notebook on his left knee beneath the table at which he was seated.

Prosecutors are expected to depict Bergdahl’s actions — made near two roads frequented by the Taliban in Paktika province, less than 20 miles from the Pakistan border — as endangering the lives of fellow soldiers and prompting a months-long manhunt that pulled valuable military resources away from other missions. Fidell has suggested Bergdahl wanted to inform senior commanders of problems in his unit — a defense that has been dismissed by his former comrades.

Army prosecutors plan to call three witnesses at the hearing: Bergdahl’s former platoon commander, company commander and battalion commander.

The first to testify, Capt. John Billing, said Thursday that Bergdahl left his rifle and other items laid out on his cot before he left. He had been a model soldier until he disappeared, carrying out tasks well with no complaint and remaining physically fit, he told a prosecutor, Maj. Margaret Kurz.

“I was in shock, m’am,” said Billings, who led Bergdahl and about 30 other soldiers at the time of the disappearance. “I was in absolute disbelief that I couldn’t find one of my men. It’s a hard thing to swallow.”

Billings described the chaotic first three weeks after Bergdahl disappeared, saying he and his soldiers completed several missions each day in hopes of finding him. The platoon, exhausted and frustrated, improvised to the point that they cut pieces from their socks and T-shirts to use as toilet paper because there was no time to get more supplies, he said.

Bergdahl planned to walk from Observation Post Mest, an austere outpost no larger than a football field, to Forward Operating Base Sharana, a larger base about 15 miles away, prosecutors said. Billings said the entire platoon already knew it was to swap with another unit and return to Sharana the following day.

Bergdahl’s disappearance prompted a hastily planned nine-man patrol led by Billings through nearby villages to look for him, the officer said. The effort left the base more vulnerable to attack, but finding Bergdahl was the top concern, he added.

The legal proceedings, known as an Article 32 hearing in the military, are expected to last at least two days and lay out new evidence against the soldier. The officer overseeing the hearing will then make recommendations to the senior officer in charge of the case, Gen. Robert B. Abrams, about whether to proceed to a court-martial trial in coming months.

The case will be closely watched in the military and on Capitol Hill, where many Republicans have blasted the Obama administration’s decision to swap Taliban officials for a soldier suspected of desertion.

Bergdahl’s legal team offered no opening statement save for a single plea that the Army release a transcript of Bergdahl’s interview with Maj. Gen. Kenneth Dahl, who carried out an investigation into the disappearance and recovery last year. Dahl will be called to testify by the defense later this week.