BBC News
The Old Bailey heard that Iranian-born Cpl Daniel James presented a charming, friendly face to colleagues as he hid what was the "height of betrayal".
In his closing speech, prosecutor Mark Dennis QC said the accused had divided loyalties and came to resent the Army.
Cpl James, 45, from Brighton, denies collecting and communicating information useful to an enemy.
The Territorial Army soldier had been working in Kabul for the head of the International Security Assistance Force (Isaf), General Sir David Richards
He was arrested in December 2006 and charged under the Official Secrets Act.
'Abuse of trust'
Mr Dennis told jurors: "The defendant was offering his services to a foreign power, contrary to the interests of Isaf and the citizens of this country – all of us."
He said Cpl James was caught "red-handed" through secret e-mail communications with Iranian military attache Colonel Mohammad Heydari.
"A British soldier setting up a clandestine relationship with a foreign power would be the height of betrayal to fellow soldiers risking their lives, day in and day out, and a grave abuse of the trust which we all, the public, put in the soldier," he said.
Having been born in Iran and still having family and property there may have given Cpl James divided loyalties, Mr Dennis said.
He added: "There may be many reasons why people over history have turned, as they undoubtedly have done.
"While keeping one face to those around them – being charitable, being friendly, engaging in social events, group charitable events – there is another face which they keep hidden from view."
'Best motives'
Colin Nicholls QC, defending, urged the jury not to jump to conclusions.
"Did Cpl James, in the latter part of 2006, offer his services to Iran and the insurgents as a spy, and supply military information?" he said.
"Or is he just some strange, complex character who was carried away by his own ideas, with the best possible motives, got out of his depth, and has been seriously misunderstood?
"It has become apparent how easy it is to fall into the trap of finding perfectly innocent events something sinister."
Cpl James has told the court that the e-mails were an attempt to set up a deal for Afghanistan to buy gas supplies from Iran.
The trial continues.
