The deal caps the surprisingly rapid and, for the military, embarrassing collapse of what once seemed a powerful case - an unraveling that began after Army prosecutors concluded that their chief witness, the captain, who had been the general's lover for three years, may have lied under oath at a pretrial hearing in January.
But the guilty pleas will end the decorated 27-year career of Sinclair, once a fast-rising star. The general, who could have faced life in prison if convicted on the sexual assault charges, will almost certainly receive a far lighter sentence, but will be required to leave the military.
The deal could still set up a showdown. Defense lawyers said military prosecutors may call the captain - as well as her parents, who are from Nebraska - as witnesses at a sentencing hearing this week, in an effort to persuade the military judge to impose tougher punishment on Sinclair.
But that would allow the general's defense team, led by a former federal prosecutor, to cross-examine the 34-year-old woman, a military intelligence officer, with what they assert are numerous instances of contradictions or deceptions discovered during a year of trial preparation. The woman already testified earlier this month about what she said were threats from Sinclair and forced oral sex, but she was not cross-examined because the court-martial was postponed.
Sinclair, 51 and married with two children, was deputy commander of the 82nd Airborne Division as well as of U.S. forces in southern Afghanistan when he was recalled in 2012. Until then, he was seen within the military as an officer who could progress to division command or higher.
The general's punishment will not be determined until a judge finishes holding the sentencing hearing. Prosecutors are expected to argue for prison time, while defense lawyers will contend that officers in similar cases have not faced jail time and have been allowed to retire at reduced rank. As one example, they cite the recent case of an Army brigadier general who lost his command and paid a $5,000 fine but was allowed to keep his rank after it was determined he had assaulted a girlfriend and committed adultery.
Defense lawyers said they and the prosecutors had also agreed to what is known as a quantum, which will serve as a cap on the amount of prison time Sinclair may face. The judge will not know what that cap is until after he determines his own sentence. The lesser of the two - the judge's sentence or the cap - will be the punishment imposed on the general, whose lawyers declined to describe the quantum in any way.
Had prosecutors proceeded with the more serious charges, which also included accusations that Sinclair performed consensual but "open and notorious sexual acts" with the captain in a parked car in Germany and on a hotel balcony in Tucson, Ariz., he could have faced the possibility of life in prison and permanent registration as a sex offender if convicted.
Defense lawyers also say Sinclair is willing to retire as a lieutenant colonel - two rungs below his current rank, and the last at which no illegal acts are alleged to have occurred - which would probably cost him more than $1 million in total retirement pay.
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Though his former lover's problematic testimony at a hearing in January shook the prosecution team, and led the chief military prosecutor to quit the case after his bosses rejected his advice to drop charges that relied solely on her testimony, Army officials say they do not question her account of the general's forcing her to perform oral sex.
But the prosecution suffered another major setback last week when the military judge, Col. James L. Pohl, ruled that the senior Army commander overseeing the case may have been wrongly influenced by political considerations when he rejected the general's earlier offer to resolve the charges by pleading guilty to lesser counts.
The judge's ruling suggested that he thought military officials, under political pressure, may have stuck with the toughest charges against Sinclair despite qualms in an effort to show new resolve against sexual misconduct.
Sinclair pleaded guilty earlier this month to charges that included adultery, requesting explicit photographs from female Army officers, possessing pornography in a combat theater and seeking a date with a lieutenant.
The new guilty pleas expected to be entered Monday include disobeying a commander's order not to contact his mistress, using demeaning language to refer to female officers and using a curse word when confronted about that conduct, and misusing his government travel charge card.