SCOTT AIR FORCE BASE, Ill. — The entire military — not just the newly reoriented Air Force — must learn to accept criticism from outside its ranks, Defense Secretary Gates said yesterday.

Speaking to about 500 airmen at this base just east of St. Louis, Mr. Gates said it is well known that the military services make good use of internal reviews of their performance in order to improve and to correct problems.

“However, I have noticed that none of the services easily accept honest criticism from outside their branch, or scrutiny that exposes institutional shortcomings,” Mr. Gates said. “This is something that must change across the military.”

Mr. Gates offered no specific examples of other branches of the military resisting outside criticism, but his remark followed his decision last week to fire Air Force Secretary Michael Wynne and the Air Force chief of staff, General Michael Moseley. Mr. Gates told his subdued audience at Scott that the shake-up was a regrettable but necessary response to Air Force failures in the performance of its nuclear mission.

The firings were prompted by the findings of an investigation by a nuclear weapons expert, Navy Admiral Kirkland Donald, who concluded that the Air Force had allowed its nuclear expertise, performance, and stewardship to erode over a period of years — despite warning signs that were not heeded by Air Force leaders.

Mr. Gates recommended that General Norton Schwartz succeed General Moseley. General Schwartz is commander of U.S. Transportation Command, whose headquarters is at Scott. Mr. Gates met briefly with General Schwartz at Scott; afterward, on his flight back to Washington, Mr. Gates told reporters that he expects President Bush to forward the Schwartz nomination to Congress when the president returns from this week’s European trip.

The brief stop at Scott was Mr. Gates’s third visit to a major Air Force base this week. He was at Langley Air Force Base, Va., on Monday after announcing his choices for replacing Mr. Wynne and General Moseley, and earlier yesterday the defense secretary spoke to several hundred airmen at Peterson Air Force Base, Colo.

At each stop, Mr. Gates mentioned that he realizes the Air Force is under strain after years of war, including its heavy involvement in the 1991 Gulf War and its patrolling of no fly zones over Iraq for the ensuing 12 years. And he repeated his promise to stop a planned further shrinkage of the Air Force; instead of reducing to 316,000 personnel as planned it will remain at today’s 330,000, Mr. Gates said.

Mr. Gates’s press secretary, Geoff Morrell, told reporters traveling with Mr. Gates that the estimated cost of forgoing further reductions would be as much as $1.4 billion a year — unrealized savings mostly from salaries and related personnel costs.

At Peterson, Mr. Gates told airmen in a private question-and-answer session that the top priority of the next Air Force civilian leader will be to restore to excellence the Air Force’s handling of its nuclear bomber, missile, and related missions, according to Mr. Morrell, who took notes at the session. Reporters were barred.

Mr. Morrell said Mr. Gates was asked by one member of his audience about the role of American nuclear weapons in today’s world. Mr. Gates replied that a major worry is that a terrorist group might acquire a nuclear weapon from a state supplier. And he said that if that happened there would be catastrophic consequences for the supplying state, according to Mr. Morrell, implying a possible American intent to retaliate with nuclear arms.

Asked later about that exchange at Scott, Mr. Gates told reporters that with regard to exactly how America would retaliate against a state supplying a nuclear weapon to a terrorist group, “It’s best to leave it ambiguous.”

Mr. Gates is adamant in cautioning against equating his firing decisions with a lack of support for the overall performance of the military, including the Air Force, which he essentially decapitated by ousting its top civilian official and its top uniformed officer on the same day last week.

But he also leaves little room for doubt that when it comes to issues like those that brought down General Moseley and Mr. Wynne — mainly a failure to reverse a record of shortcomings in the Air Force’s nuclear mission — he will not hesitate.

“There is simply no room for error in this mission,” he told airmen at Scott. “Nor is there, unfortunately, any room for second chances.”

The chairman of the Joint Chiefs of Staff, Admiral Mike Mullen, was asked yesterday morning about Mr. Gates’s quick hand at replacing top officials, and he recalled that Mr. Gates, in his very first meeting with senior military leaders, talked about accountability.

“He’s a leader who decentralizes control, but then he would hold leaders accountable,” Mr. Mullen told reporters at a breakfast meeting.

Mr. Mullen added that he endorsed Mr. Gates’s leadership changes, and that the message to be taken away from it all is “how important it is to ensure that the nuclear mission is well tended to. We have been slipping for a significant period of time, and we need to arrest that.”

General Schwartz has a varied background that includes numerous assignments — and a sterling reputation — in Air Force special operations, a highly demanding and often secretive part of the service. He commanded the 16th Special Operations Wing at Hurlburt Field, Fla., in 1995-97, then commanded the Pacific region’s special operations forces for a year and a half, and spent nine months in 2000 as deputy commander of U.S. Special Operations Command at Tampa, Fla.