Military prosecutors have said US soldier Bradley Manning "systematically harvested" a vast trove of secret documents to share with Wikileaks.
At the start of Pte Manning's court martial, a prosecutor said Osama Bin Laden had received leaked information.
But defence lawyers said Pte Manning, 25, was young and naive when he shared the files with the anti-secrecy site.
He has not denied his role in the leak, and faces up to life in prison if convicted of aiding the enemy.
Earlier this year, Pte Manning pleaded guilty to 10 of the 22 charges against him related to the leaks, but not to the most serious charge.
The Manning-Wikileaks case is considered the largest-ever leak of secret US government documents. Prosecutors say the disclosures harmed US national interests, while Pte Manning's supporters say he is a whistle-blowing hero.
In opening statements on Monday at a military courtroom in Fort Meade, Maryland, prosecutor Capt Joe Morrow called the case an example of what happened "when arrogance meets access".
'Harvest' of documents
Capt Morrow argued the case was not about a whistleblower's leak of targeted information.
"This, your honour, this is a case about a soldier who systematically harvested hundreds of thousands of documents from classified databases and then dumped that information on to the internet into the hands of the enemy," he said.
According to the prosecutor, Pte Manning used his military training to gain the notoriety he craved and attempted to hide what he had done at every step of the process.
He said he would introduce evidence Osama Bin Laden himself had gained access to some of the Wikileaks information - and had put it to use.
Prosecutors plan to introduce blog entries, a computer, a hard drive and a memory card as evidence against Pte Manning. The military prosecutors will also call witnesses to describe his training and his deployment to Iraq.
In an opening statement, Pte Manning's lawyer David Coombs said he was "young, naive and good-intentioned" when he arrived in Iraq.
But in late 2009, after an Iraqi died in an attack, he grew disillusioned after seeing his comrades celebrating because no US soldiers had been hurt.
After that incident, Pte Manning began collecting information he thought would "make the world a better place" if public.
"He believed this information showed how we value human life," Mr Coombs said. "He was troubled by that. He believed that if the American public saw it, they too would be troubled."
The defence lawyer argued that Pte Manning was "selective" in his choice of the hundreds of millions of documents he had access to.
The prosecution's opening arguments directly relate to the most serious charge against Pte Manning: aiding the enemy. To obtain a conviction, prosecutors must prove Pte Manning acted with intent to aid the enemy and knowingly gave such adversaries US intelligence information.
The BBC's Mark Mardell says the prosecution's argument - that releasing such information on to the internet counts as aiding the enemy - has serious implications for anyone leaking classified information in the future.
Our correspondent adds the military will aim to show the information was of "great value" to US enemies, but supporters argue all Pte Manning did was make public what should never have been private.
Pte Manning, who was arrested in May 2010 while serving in Iraq, has not denied leaking the documents.
He told a pre-trial hearing in February he divulged the documents to spark a public debate on the role of the US military and foreign policy.
However, prosecutors argue the leaks damaged national security and endangered American lives.
One of the leaked videos shows graphic footage of an Apache helicopter attack in 2007 that killed a dozen people in Baghdad, including a Reuters photographer.
Other documents leaked included thousands of battlefield reports from Afghanistan and Iraq, as well as secure messages between US embassies and the state department in Washington.
Whatever prison sentence Pte Manning receives will be reduced by 112 days, after a judge ruled he had suffered unduly harsh treatment during his initial detention following his arrest.
Assange asylum talks
The soldier chose to have his court martial heard by a judge instead of a jury. It is expected to run all summer.
Judge Col Denise Lind ruled in May she would close parts of the trial to the public to protect classified material.
Meanwhile, the UK government said on Sunday it was considering a request from Ecuador to hold talks on the future of Wikileaks founder Julian Assange.
Mr Assange has lived in the Ecuadorean embassy in London for a year, having been granted political asylum there.
He faces extradition to Sweden over sex allegations, which he denies.