GUANTANAMO BAY NAVAL BASE, Cuba — A smiling Omar Khadr appeared in a suit and tie Tuesday and greeted prospective jurors at the start of his trial, billed by defense lawyers as the first war-crimes prosecution of a child soldier since World War II.

Khadr, the Toronto-born son of an alleged al-Qaida financier, is the youngest prisoner at Guantanamo and the only remaining Westerner.

He was 15 when he allegedly hurled a grenade that killed a U.S. Delta Force soldier during a firefight in Afghanistan in 2002.

The trial is the first in the tenure of President Obama, whose administration is weighing whether to use the embattled Guantanamo courts as a possible venue for dozens of other detainees – including “high-value” figures such as the self-professed mastermind of the Sept. 11, 2001 attacks and four alleged accomplices.

The Khadr case has been held up by legal wrangling since he made his first appearance in the courthouse overlooking the Caribbean Sea in 2006.

The Canadian, now a bearded and broad-shouldered man of 23, traded his prison jumpsuit for a gray suit before the start of jury selection. As his Pentagon-appointed attorney introduced him to the pool of 15 U.S. military officers, Khadr stood and said to them in English: “How are you?”

Once a panel of at least five officers is seated, opening arguments will begin. The trial expected to last roughly three weeks.

Defense lawyers say Khadr was a victim, forced into war by a family with close ties to Osama bin Laden. Khadr’s father, Ahmed Said Khadr, was an Egyptian-born Canadian citizen and alleged militant killed in 2003 when a Pakistani military helicopter shelled the house where he was staying with senior al-Qaida operatives.

“He’s not a real Taliban warrior. He’s a kid who was put in an unfortunate situation,” said Dennis Edney, a Canadian lawyer for Khadr.

Khadr has pleaded not guilty to five charges, including murder.