Correction: An earlier version of this article incorrectly described the reason that Julian Assange is being sought by authorities in Sweden. He is wanted for questioning in connection with sex crimes he has been accused of committing, but he has not been charged with any crimes.

For more than three years, Army Pfc. Bradley E. Manning has been in detention awaiting court-martial for the largest leak of classified documents in U.S. history. When his trial finally opens Monday, it will be the latest and most high-profile in a series of leak prosecutions brought by the Obama administration.

Manning, a boyish-looking 25, is accused of passing more than 700,000 government and military files to the anti-secrecy Web site WikiLeaks. The material, which was widely disseminated, included videos of airstrikes that killed civilians, sensitive diplomatic cables and military reports from Iraq and Afghanistan.

He faces 22 charges, including a military charge of aiding the enemy, which could send him to prison for life without parole. He is also charged with violating the Espionage Act, a 1917 law created to try spies and traitors, which carries severe penalties.

There was a crowd of more than thirty Manning supporters protesting at the main gates of Fort Meade in Maryland in the rain to mark the start of the trial. Protesters held signs like “free Bradley Manning” and “Does America Have a Conscience.”

In pretrial proceedings, Manning admitted leaking the material, saying he intended to “spark a domestic debate over the role of the military and our foreign policy in general.” He has offered to plead guilty to 10 lesser charges relating to the misuse of classified information, which could send him to prison for 20 years.

The prosecution’s insistence on proceeding with the two most serious charges, with their harsh penalties, has led even some critics of Manning’s actions to question whether the government is going too far.

Elizabeth Goitein, co-director of the Brennan Center for Justice’s Liberty and National Security Program, called Manning’s leaks “reckless” and “a data dump.” But she said that “he is not an enemy of the state” and that putting him behind bars for life “is overreaching.”

Michael Ratner, president emeritus of the Center for Constitutional Rights, said he was shocked the government proceeded after the plea offer. “It is like hitting Manning with a sledgehammer,” he said. “They have him for 20 years, and then they go for life. Twenty years is enough for a pound of Manning’s flesh.”

For its part, the prosecution said Manning must be held accountable. “Pfc. Manning was a U.S. analyst who we trained and trusted to use multiple intelligence systems . . . and he used that training to defy our trust,” Army Maj. Ashden Fein, a prosecutor, said in one pretrial hearing.

Manning, he said, “knowingly engaged in a six-month-long criminal enterprise of harvesting classified information” to send to WikiLeaks, “while knowing and understanding that enemies would have access to the information.” The enemies, Fein said, include al-Qaeda.

The differences of opinion over Manning, a former Army intelligence analyst, extend to the core of the case. Since his arrest in Iraq in May 2010, he has become a polarizing figure of international stature. Supporters see him as a heroic whistleblower; critics view him as a traitor who harmed the nation and put lives at risk. He is even a figure in a new documentary film that focuses on WikiLeaks founder Julian Assange, who is holed up in the Ecuadorean Embassy in London to avoid questioning in Sweden about sex crimes that two women have accused him of committing.

The treatment of Manning at times over the past three years — his clothes were taken away, and he was held in isolation — has led to international protests. And the military’s decision to withhold judicial rulings from the public has led to lawsuits.

Manning’s supporters held a protest march and rally outside Maryland’s Fort Meade on Saturday. Protesters carried signs, some reading “Free Bradley” and “Bradley Manning: Jailed for exposing war crimes.”

Now, after months of pretrial hearings, motions and disputes over the use of classified information and public access to proceedings, the trial will begin in a spare courtroom behind Fort Meade’s guarded gates.

Barring a last-minute plea deal, the court-martial will be one of the biggest military trials since the cases stemming from the My Lai massacre in the Vietnam War. The process is expected to last at least 12 weeks, and the government plans to call 150 witnesses — 24 of them in partially closed sessions to prevent what the judge, Army Col. Denise Lind, called “spillage of classified information.”

Manning opted to be tried by a judge rather than a jury of military members. Lind, who has presided over the pretrial hearings, has granted Manning 112 days’ credit toward his sentence if he is convicted, to compensate for his treatment.

Analysts are divided on how tough a challenge prosecutors face to win convictions on the most serious charges. The Brennan Center’s Goitein said that under a ruling by Lind, prosecutors will have to prove only that Manning had “reason to believe” that the documents disclosed could be used to harm the United States or to aid a foreign power. They need not prove that he intended to harm the United States.

“I suspect that the government can meet this burden on at least some of the counts,” Goitein said.

But some former prosecutors said it could be difficult to prove intent to harm the United States.

“A lot of times, you think something is damaging,” said Baruch Weiss, a former federal prosecutor and an expert on the Espionage Act, “and the reality proves to be otherwise.”

Even some of Manning’s critics argue that his disclosures served the public interest in some instances. They cite his leak of a video showing an Apache helicopter assault in Baghdad that killed Iraqi civilians and two journalists, the disclosure of risk assessments for several dozen Guantanamo Bay detainees showing no record of why they were transferred to the military prison, and accounts of unreported civilian deaths in Afghanistan and Iraq.

On the other hand, they argue that some disclosures put foreign relations or even lives at risk with no obvious benefit to public debate. One example is the disclosure of the names of Afghan informants cooperating with allied forces, which put them in danger and forced steps to find and protect them.

The secrecy surrounding the court-martial, while not unusual in military cases, has frustrated observers. Lind has read lengthy rulings from the bench but not made them available. More than 30,000 pages of classified and unclassified motions have been produced in the case, but none of the unclassified material has been released.

Even documents that Manning has admitted providing to WikiLeaks, and which have been published worldwide, remain classified and cannot be mentioned in open court.