WASHINGTON — A weak command structure and a climate of fear among female personnel created the conditions that led to widespread instances of sexual assault of Air Force recruits by their instructors at Lackland Air Force Base in Texas, senior Air Force commanders said Wednesday.

On the eve of a Pentagon announcement that it will lift the ban on women in combat, Gen. Mark A. Welsh III, the Air Force chief of staff, told the House Armed Services Committee that there was poor oversight of instructors at the Air Force’s version of boot camp at Lackland, and acknowledged that “weaknesses developed in each one of our institutional safeguards” that led to a poisonous culture in which the instructors believed they could easily get away with repeatedly preying on young woman recruits.

The Lackland case, allegedly involving 32 instructors who took advantage of their power over as many as 59 recruits, is one of the largest sex scandals in the military since the Tailhook episode of the early 1990s, and has come at a time when the problems of sexual assault, harassment and abuse have become major issues for the military.

Last year, Defense Secretary Leon E. Panetta acknowledged that the number of sexual assaults in the military is probably far higher than official statistics have shown.