Bradley Manning's letter, entitled The Next Stage of My Life, was signed with a handwritten signature in the name of Chelsea Manning.
The letter was broadcast on NBC's Today show as Manning's lawyer revealed the soldier's desire to live life as a woman.
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The former reality TV star will follow in the footsteps of Abraham Lincoln and John F Kennedy when he takes his oath later on Friday.
The President-elect's comments on Twitter will come under even closer scrutiny when he takes office on Friday.
Cold, quiet and calm