Bradley Manning saw military videos of apparent war crimes and released them to the media after the military refused to take action. He found documents that he felt described other war crimes or cases of our government lying to the public. To publicize such wrongdoing, he leaked a set of such documents (all classified less than top secret) to an organization to provide them to the public, resulting in thousands of news articles based on those documents in major news organizations around the world.
Bradley Manning has technically violated the law in exposing more serious crimes and helping the American public understand what the government is doing in its name. He has already served more than 3 years in prison for his selfless acts.
Pardon Manning now; he has been punished enough.