Chris Beck spent 20 years as an elite US Navy Seal, often operating in secret behind enemy lines. But the highly decorated serviceman was always hiding a deeper, personal secret - since early childhood, he felt he was a female born into a male body.

As a Navy Seal, Chris Beck's world was tough, macho, sometimes violent. He took part in covert missions from the Pacific Ocean to the Middle East and fought alongside members of Britain's SAS on the Shatt al-Arab waterway near Basra during the 2003 war in Iraq.

But in February, more than a year after retiring from the US Navy, he replaced the photograph on his LinkedIn profile with one of a tall brunette in a white blouse smiling in front of the Stars and Stripes and wrote "I am now taking off all my disguises and letting the world know my true identity as a woman." Chris had become Kristin.

As she awaited the reaction from her former brothers-in-arms, Kristin knew there was no going back from her decision to go public.

US Navy Seals are sent on some of the most difficult and dangerous military operations in the world. One of Kristin Beck's former units, the US Naval Special Warfare Development Group - also known as Seal Team 6 - went on to carry out the mission that killed Osama Bin Laden in Pakistan in May 2011.

I had a secret life with my female identity [and] I had my secret life with the Seal teams

The Navy Seal code demands that fighters uphold the unwavering values of loyalty, integrity and trust. Kristin feared some fellow Seals would accuse her of dishonouring that code by coming out as transgender.

While some did find the decision difficult to accept, the response was overwhelmingly positive. "A lot of them said 'Kris - I really don't understand what you're going through but I know where you've been,'" she told the BBC.

"My Seal team brothers said, 'you stood the watch in the field for 20 years and you did a great job. I don't understand it one bit but I support you 100% and I hope I can learn more about this and see you at the next reunion.'"

Knowing the news would eventually spread beyond the tightly-knit Navy Seal community, Kristin decided to tell her story before someone else did.

The military needs to get past gender and look at people like me as a person, not just as a male or female

She co-wrote a book, Warrior Princess: A US Navy Seal's Journey to Coming Out Transgender, with Anne Speckhard, a professor of psychiatry at Georgetown University in Washington DC. It charts her childhood in a religious and socially conservative household, her attempts to suppress her gender identity by secretly buying and then discarding women's clothes, and her two failed marriages.

"I was trying to live three lives," Kristin says. "I had a secret life with my female identity, I had my secret life with the Seal teams and then I had my home life and what I would show my wife and children or parents and friends.

"People would see snippets of the real me but for the most part nobody really got to know me."

The rapid and aggressive tempo of special forces operations following the attacks of 11 September 2001, combined with an emotional life which she says was "totally squashed", took its mental toll on Kristin and she developed post traumatic stress disorder (PTSD).

She says that for years she dealt with the psychological impact of "so much death, so much pain" through "beer, motorcycles, more beer".

Yet she says coming out as transgender has had a "dramatic impact" on her PTSD symptoms. "I'm not as angry and I sleep better just because I'm happier," she says.

US Navy Seals US President John F Kennedy announced the creation of a Navy special operations force in 1961. The first teams were formed "to conduct counter guerilla warfare and clandestine operations" The name Seals refers to where the teams operate - on sea, air and land The maximum age to apply is 28. The gruelling training to become a Seal is said to be the toughest of any special forces in the world - the drop-out rate is believed to be about 80% Navy Seals have been involved in some of the US military's most dangerous operations including in Vietnam, Somalia, Iraq and Afghanistan Seals took part in the raid on Bin Laden's compound in Abbottabad, Pakistan, in which the then US public enemy number one was killed

"So many people have said, 'Kris, for the first time in my life I've actually seen you smile.'"

The repeal of the "don't ask, don't tell" policy in 2011 ended the ban on openly gay men and women in the US military. That shift doesn't apply to transgender individuals, however, who can still be discharged if they are found out.

Kristin Beck believes that policy could, and should, change. She proposes allowing transgender service personnel to undergo gender reassignment in a military setting in return for extending their time in uniform once the process is complete.

"It's a human condition," she says. "The military needs to get past gender and look at people like me as a person, not just as a male or female, and understand that I can still do a great job. I may not be able to do all the jobs I was doing before but I can do something else. I could be an intelligence analyst or a security officer at a checkpoint.

"None of us are perfect. I'm not Conan the Barbarian and I'm not Barbie. We're all different."

Kristin says she would have preferred to go through her continuing gender transition in private, rather than in the glare of attention that has inevitably followed the publication of her book.

She says, however, that she is approaching her new role as an unofficial spokeswoman for the transgender community with the same "warrior spirit" - a sense of leadership and commitment to duty despite the odds - that defined her military career.

"I think I've saved some lives. I've had some heart-wrenching emails from people who are caught up in pain and prejudice and that does make it worthwhile," she says.

"I've also had emails from straight men who have said 'thank you for your service for our country. I never understood what this was but now I do.'"

"Fear of the unknown is the biggest problem and I think reading my book has helped break down that fear for many people," Beck says. "I'm not going to hurt anyone and I'm not contagious. I'm just me."

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