Sinclair, 51, has pleaded not guilty to eight criminal charges including forcible sodomy, indecent acts, violating orders and conduct unbecoming an officer. He faces a maximum sentence of life in prison at a court-martial scheduled to begin March 3.
While he denies the most serious allegation that he physically forced a female captain under his command to perform oral sex, the married father of two concedes he carried on a three-year extramarital affair with the junior officer. That admission alone will almost certainly end his 28-year Army career, as adultery is a crime under military law.
The female captain who made the initial complaint admits to having consensual sex with Sinclair on numerous occasions, both before and after the alleged assaults.
Prosecutors also allege the general had inappropriate contact with other women, including female officers expected to testify that he asked them to provide nude photos of themselves.
At a pre-trial hearing Tuesday, the Army captain testified about a new discovery in the case. The woman said that in early December, she'd become frustrated by a meeting with government lawyers about the upcoming trial.
Through tears, she said she felt nervous and concerned about the trial, and that made her face the cleanup of a room in her Arizona home that she said had been a total mess. After dumping a box of electronics, she said she came across an old broken iPhone 3 that she said hadn't been used since 2010 because the screen was broken, and thought it could have info the government would want to see for the trial.
She testified she left it charging overnight, and the next morning, Dec.10, she turned it on, and found text and voice messages under the alias "Nathan" she'd given the general.
But Sinclair's counsel challenged the woman's version of events. A computer and cell phone forensics expert testified for the defense that the phone was turned on several times between 2010 and December.
Sinclair's lawyer alleges that proves the woman lied.
"This morning's hearing was a game-changer. Not only did the captain withhold and tamper with material evidence. She also misled prosecutors and gave false, sworn testimony to the court. The government is effectively left with a chief witness who has perjured herself," offered Richard Scheff.
The contents of the phone messages have not been revealed.
The judge, at the request of the defense, has given the government two weeks to examine other electronic devices in the accuser's possession to determine whether any of them were used during the dates of her relationship with Sinclair.
Prosecutors portray Sinclair as a sexual predator who abused his position of authority to prey on a subordinate trained to follow his orders, threatening to kill her and her family if she told anyone of their relationship.
Sinclair's defense lawyers have suggested he is the victim, both of a jealous ex-lover and overzealous prosecutors facing intense pressure from top military and political leaders to send a message that sexual misconduct will not be tolerated. They say the evidence against him is weak - a case that in the past might have been resolved with a quiet reprimand and early retirement.
It has been a steep fall for a man once considered a rising star among the Army's cadre of trusted battlefield generals. As deputy commander of the 82nd Airborne, Sinclair oversaw 22,000 troops until he was abruptly sent home from Afghanistan last year and criminally charged.
AP writer MICHAEL BIESECKER contributed to this report.