A rise in rapes -- along with a wave of high-profile scandals in recent weeks -- has put military leaders on the defensive.
"You need to do more and you need to do it much faster," Senator Mark Udall told the chiefs at a packed hearing of the Senate Armed Services Committee.
The top officers, who made an unusual joint appearance before the panel, seemed intent on heading off proposals in Congress that would alter legal traditions the military holds dear, including rules that allow commanders to decide if a case should go to trial.
"I understand the credibility of the armed forces, the credibility of the army are at stake," said General Ray Odierno, the Army's chief of staff.
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"But we cannot simply legislate our way out of this problem."
Several lawmakers rejected the chiefs' stance, saying fundamental changes were needed to open the way to more prosecutions of sexual predators.
Senator Claire McCaskill of Missouri sharply criticized the four-star generals, saying the military misunderstood the problem -- lumping together rape and other crimes with sexual harassment at the work place.
The force needs to better document sex crimes, ramp up prosecutions and make it easier for rape victims to come forward, she said.
Sexual assault victims in the military are "afraid to report" the crime to commanders who hold sway over their careers, said Senator Kirsten Gillibrand of New York.
"They think their careers will be over. They fear retaliation. They fear being blamed," she said.
The Democrat is pushing for a bill that would have military prosecutors decide whether a sexual assault or other criminal case should go to trial instead of a unit commander, who she said could lack impartiality.
But General Martin Dempsey, the military's top-ranking officer, said restricting a commander too severely could undermine an officer's ability to ensure discipline in a unit.