FORT BRAGG, N.C. - As his court-martial got underway, an Army general accused of sexual assault pleaded guilty to three lesser charges Thursday, hoping his admission will strengthen his case by limiting some of the salacious evidence against him.

Brig. Gen. Jeffrey A. Sinclair is accused of forcing a female captain to twice perform oral sex and threatening to kill her family if she told anyone about their three-year affair.

Sinclair, 51, still faces five charges, including sexual assault, in his trial before a jury of five generals. The former deputy commander of the 82nd Airborne could be sentenced to life in prison if convicted on the most serious charges. Opening statements were expected Friday.

The case against Sinclair, believed to be the most senior member of the U.S. military ever to face trial on sexual assault charges, comes as the Pentagon grapples with revelations of rampant rape and sexual misconduct within the ranks. The U.S. Senate is expected to vote Thursday on legislation that would strip senior military commanders of their authority to prosecute rapes and other serious offenses, a measure firmly opposed by the Pentagon.

The general pleaded guilty to having improper relationships with two other female Army officers and to committing adultery with the primary accuser, his mistress, which is a crime in the military. He also admitted to violating orders by possessing pornography and to conduct unbecoming of an officer and a gentleman.

Sinclair's lawyer Richard Scheff said before the plea that his client was taking responsibility for his actions, but also strengthening his legal position. By admitting guilt on the three charges for which there is the strongest evidence, the married father of two hoped to narrow the focus of the trial to charges that rely heavily on the testimony and credibility of his former mistress.

In pleading guilty to possessing a hoard of porn on his laptop in Afghanistan, a violation of orders for soldiers in the socially conservative Muslim country, Sinclair's defense hopes to limit the ability of prosecutors to use those graphic images to shock the jury.

Prosecutors also have evidence Sinclair asked two female officers to send nude photos of themselves to him. By conceding his quilt, the defense lessens the relevance of the messages they exchanged. The primary accuser is the only one alleging assault.

"The government now has a big problem," Scheff said in an email before the plea. "It took pathetically weak assault charges and put a fancy wrapper around them. We just tore the wrapper off. The prosecution team no longer gets to distract us with salacious details about acts that aren't even criminal in the civilian world. All they're left with is a crime that never happened, a witness who committed perjury, and a pile of text messages and journal entries that disprove their claim."

The defense will present evidence that the female captain lied under oath during a pretrial hearing in January about her handling of old iPhone containing messages between her and the general. Lawyers for Sinclair have painted the woman as a scorned lover who only reported the sexual assault allegations after the general refused to leave his wife.

The Associated Press generally does not identify those who say they were sexually assaulted.

The captain testified that on Dec. 9, shortly after what she described as a contentious meeting with prosecutors, she rediscovered an old iPhone stored in a box at her home that still contained saved text messages and voicemails from the general. After charging the phone, she testified she synced it with her computer to save photos before contacting her attorney.

However, a defense expert's examination suggested the captain powered up the device more than two weeks before the meeting with prosecutors. Three additional experts verified the finding.

During a pretrial hearing this week, a top Pentagon lawyer testified that the lead prosecutor assigned to the case for nearly two years, Lt. Col. William Helixon, had urged that the most serious charges against Sinclair be dropped after he became convinced the captain had lied to him about the cellphone. Helixon was overruled by his superiors and then removed from the case last month, after suffering what was described as a profound moral crisis that led to his being taken to a military hospital for a mental health evaluation.

The case now heads to trial with a new lead prosecutor, Lt. Col. Robert Stelle, who said in court this week he doesn't care what his predecessor thought about the weakness of the evidence.

It is highly unusual for an officer of flag rank to face criminal prosecution, with only a handful of cases in recent decades. Under military law, an officer can only be judged at trial by those of superior rank.