Aug. 13, 2010  -- Katherine Miller is the ninth-ranked Cadet in her class of 1,157 at the United States Military Academy. She has a grade point average of 3.829 out of 4.0, and has scored as high as 367 on the military's fitness test, beating the theoretical maximum score of 300 points. She has completed U.S. Army Airborne School and written for a Washington Post blog.

Yet Miller will not enroll in West Point for her third year, will not graduate, join the Army and serve her country, and it has nothing to do with a disciplinary infraction or lack of qualifications. Instead, it is her sexual orientation.

Miller, 20, submitted her letter of resignation to her superiors and in it, revealed to them that she is a lesbian. She has resigned because of the military's "Don't Ask, Don't Tell" policy, or DADT, which forbids the military from asking a servicemember about her sexual orientation and calls for a servicemember to be discharged if she discloses that she is homosexual.

"At present, I find military service to be incompatible with personal values," Miller wrote in a draft of the letter, which was leaked to the media. She also wrote that she was not pressured or encouraged to resign, and came to the decision on her own.

Miller said in the letter that she can no longer bear to attend West Point, where the military's policy has forced her to conceal a portion of her identity, fabricate a straight dating history to appease her peers and stand by while others made derogatory comments about gays. She said she routinely endured sexual harassment for fear of being outed.

"In short, I have lied to my classmates and compromised my integrity and my identity by adhering to existing military policy," Miller wrote.

Sue Fulton, a West Point graduate, former Army captain, and communications director of Knights Out, a group of gay and lesbian West Point graduates and allies, said being homosexual in the military is extremely difficult. Knights Out provides advice for closeted cadets, but also works to show the military that gays and lesbians can and do serve effectively and provide a positive impact on the military. They are also actively engaged in working to repeal DADT.

"It's a constant struggle. There are constant compromises to your integrity when you can't be honest about who you are," Fulton told ABCNews.com.

"I know that Katie agonized over this because she loves the military," she said. "I think it would be fair to say that she is sad, but she is very clear on this decision."

Lesbian Cadet Quits West Point over 'Don't Ask, Don't Tell'

The timing of the decision was important, because cadets are required to make a commitment before their junior year to serve in the military after graduation, or they can face penalties like having to pay back the government for their education, which is usually provided for free in exchange for military service.

"To make a commitment of ten years, not knowing for sure that she wouldn't be hiding for the next decade, I think was just too much," Fulton said.

Lt. Col. Brian Tribus, Director of Public Affairs and Communications at West Point, confirmed in an e-mail to ABCNews.com that Miller's resignation has been granted, and was in the process of leaving the academy on Friday afternoon.

Miller was not available for comment further.

Before coming out officially, a person believed to be Miller posted about her experience as a closeted lesbian while at West Point on Velvetpark, an online social network for lesbians, under the name "Private Second Class Citizen."

Private Second Class Citizen posted a series of three "lessons" on West Point lesbian culture. In them, she described the secret network of lesbians at the Academy and other classmates' attempts to find out if she was gay. She and her lesbian friends referred to each other as "bros," she wrote. When one of them suspected another classmate was lesbian, she would ask the others, "Yo, is that girl family?" Private Second Class wrote.

In her third "lesson," dated August 5, she revealed that she intended to return to West Point this fall only to come out to her superiors and resign from the Academy. In an earlier post, she wrote, "How is one supposed to wholeheartedly commit to an institution that so blatantly discriminates against them?"

Though she wrote that there was an underground network of lesbians at West Point, it seems few choose Miller's path.

Lesbian Cadet Quits West Point over 'Don't Ask, Don't Tell'

"At this time, we are unaware of any other cadets that have cited the DADT policy as a reason for resignation," Lt. Col. Tribus wrote.

Since Miller resigned before the start of her junior year, she faces no penalty or obligation to the government. Miller has been admitted as a transfer student to Yale University.

"We wish her well at Yale," Lt. Col. Tribus wrote.

Miller wrote that she will continue to fight the policy as a civilian. Miller, who is majoring in sociology, wrote that she has studied the policy and scrutinized it from an academic standpoint. She has written articles and presented her research on the subject at conferences.

"Through my academic endeavors I cannot objectively rationalize the military's policy concerning homosexuality," she wrote. "I intend for my resignation to offer a concrete example of the consequences of a failed law and social policy."

Fulton said neither she nor Knights Out recommend that Cadets resign as Miller did.

"I'm disappointed that she's resigning because I think she's exactly the kind of leader that our soldiers need, but I understand her reasons," Fulton said.

"This is a tragedy for West Point."

If cadets like Miller feel they must resign, Fulton said, she hopes it will energize others to work to change the policy and prevent the military from losing other capable leaders.

Miller said she will rejoin the military if the policy is repealed.

"Service to others remains a core value to me, and I intend to repay my country for all the freedoms I enjoy as a U.S. citizen," Miller wrote. "I have made it a personal responsibility to aid in the improvement of military personnel policy -- as civilian or as soldier -- in order to enhance force readiness and improve national defense capabilities."