WASHINGTON >> The Air Force took the extraordinary step Thursday of firing nine midlevel nuclear commanders and announcing it will discipline dozens of junior officers at a nuclear missile base, responding firmly to an exam-cheating scandal that spanned a far longer period than originally reported.

A 10th commander, the senior officer at the base, resigned and will retire from the Air Force.

Air Force officials called the discipline unprecedented in the history of America’s intercontinental ballistic missile force. The Associated Press last year revealed a series of security and other problems in the ICBM force, including a failed safety and security inspection at Malmstrom Air Force Base, Mont., where the cheating occurred.

In an emotion-charged resignation letter titled “A Lesson to Remember,” Col. Robert Stanley, who commanded the 341st Missile Wing at Malmstrom, lamented that the reputation of the ICBM mission was now “tarnished because of the extraordinarily selfish actions of officers entrusted with the most powerful weapon system ever devised by man.”

Stanley, seen as a rising star in the Air Force, had been nominated for promotion to brigadier general just days before the cheating scandal came to light in January. Instead he is retiring, convinced, as he wrote in his farewell letter Thursday, that “we let the American people down on my watch.”

Separately, another of the Air Force’s nuclear missile units — the 90th Missile Wing at F.E. Warren Air Force Base, Wyo. — announced that it had fired the officer overseeing its missile squadrons. It said Col. Donald Holloway, the operations group commander, was sacked “because of a loss of confidence in his ability to lead.”

The 90th Missile Wing offered no further explanation for Holloway’s removal and said it “has nothing to do” with the firings announced by the Air Force in Washington.

Together, the moves reflect turmoil in a force that remains central to American defense strategy but in some ways has been neglected. The force of 450 Minuteman 3 missiles is primed to unleash nuclear devastation on a moment’s notice, capable of obliterating people and places halfway around the globe.

In a bid to correct root causes of the missile corps’ failings — including low morale and weak management — the Air Force also announced Thursday a series of new or expanded programs to improve leadership development, to modernize the three ICBM bases and to reinforce “core values” including integrity.

Air Force Secretary Deborah Lee James, the service’s top civilian official, told a Pentagon news conference that a thorough review of how testing and training are conducted in the ICBM force has produced numerous avenues for improvements.

“We will be changing rather dramatically how we conduct testing and training going forward,” while ensuring that performance standards are kept high, James said.

James had promised to hold officers at Malmstrom accountable once the cheating investigation was completed and the scope of the scandal was clear. None of the nine fired commanders was directly involved in the cheating, but each was determined to have failed in his or her leadership responsibilities.

Lt. Gen. Stephen Wilson, commander of Air Force Globe Strike Command, said investigators determined that the cheating, which officials originally said happened in August or September last year, began as early as November 2011 and continued until November 2013. Wilson is responsible not only for the ICBM force but also the B-52 and B-2 bombers that are capable of launching nuclear attacks.

The cheating involved unauthorized passing of answers to exams designed to test missile launch officers’ proficiency in handling “emergency war orders,” which are messages involving the targeting and launching of missiles.

When the cheating was first revealed, Air Force leaders condemned it as violating the most basic Air Force values. They also suggested that it reflected an unhealthy pressure from commanders to achieve perfect test scores. The tests are one part of a training and evaluation system that is supposed to ensure that every one of the more than 500 missile launch officers is fully proficient.

Nine key commanders below Stanley were fired, including the commanders of the 341st Wing’s three missile squadrons, each of which is responsible for 50 Minuteman 3 nuclear missiles.

Also sacked were the commander and deputy commander of the 341st Operations Group, which oversees all three missile squadrons as well as a helicopter unit and a support squadron responsible for administering monthly proficiency tests to Malmstrom’s launch crews and evaluating their performance.

Members of all three missile squadrons were implicated in the cheating, either by providing or receiving test answers or knowing about the cheating and not reporting it.

No generals are being punished. Maj. Gen. Michael Carey, who was fired last October as commander of the 20th Air Force, which is responsible for all three 150-missile wings of the ICBM force, is still on duty as a staff officer at Air Force Space Command but has requested retirement; his request is being reviewed.

Carey was fired after a military investigation determined that he had engaged in inappropriate behavior while leading a U.S. government delegation to a nuclear security exercise in Russia last summer. He was replaced by Maj. Gen. Jack Weinstein.

The cheating at Malmstrom was discovered in early January during the course of an unrelated drug investigation that included two launch officers at Malmstrom and others at several other bases; the drug probe is continuing.

A total of 100 missile launch crew members at Malmstrom were identified as potentially involved in the cheating, but nine were cleared by investigators. Another nine of the 100 are being handled separately by the Air Force Office of Special Investigation; eight of those nine involve possible criminal charges stemming from the alleged mishandling of classified information.

Of the remaining 82 officers, an estimated 30 to 40 are eligible to be retrained and returned to duty on the missile force; the rest face unspecified disciplinary action that could include dismissal from the Air Force, officials said. Of the 82 facing disciplinary action, 79 are still at Malmstrom; the three others had moved to other bases or left active duty.

Wilson, the Global Strike Command chief, said the changes in training and testing will be far-reaching.

“We’re not just putting a fresh coat of paint on these problems,” he said. “We’re taking bold action.”

After the cheating was announced in January, Defense Secretary Chuck Hagel launched a pair of in-depth reviews of the nuclear forces to determine why the ICBM force has suffered so many setbacks over the past year.

Hagel said he his goal was to restore public confidence in the nuclear force.