(CNN) It must have stuck in President Barack Obama's craw to deliver a win for WikiLeaks.

But that is effectively what he had to do to commute the 35 year sentence of Pvt. Chelsea Manning, who was convicted of committing one of the biggest and most embarrassing leaks of classified information in US history.

The move came with the hours fast running down on Obama's presidency and rocked the political, intelligence and military intelligence establishments in Washington. And it will now rest forever on the 44th President's legacy as one of the most controversial moments of his tenure, judging by the furious bipartisan reaction to his decision to free someone regarded by many in Washington as as a traitor.

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His decision to announce the commutation on the eve of his final presidential news conference on Wednesday indicates he wanted to explain the reasons for his decision to the American people in person. That opportunity may go some way to ensuring that the move does not sow controversy at the start of his post-presidency, as Bill Clinton's controversial pardon for fugitive financier Marc Rich did in 2001.

Senior officials told CNN that Obama decided to act because Manning had expressed remorse and responsibility for her actions, and she had already served six years of a long sentence.

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