Defense Secretary Ashton B. Carter said Thursday that he is opening all jobs in combat units to women, a landmark decision that would for the first time allow female service members to join the country’s most elite military forces.

Women will now be eligible to join the Navy SEALs, Army Special Forces and other Special Operations Units. It also opens the Marine Corps infantry, a battle-hardened force that many service officials had openly advocated keeping closed to female service members.

“There will be no exceptions,” Carter said. “This means that, as long as they qualify and meet the standards, women will now be able to contribute to our mission in ways they could not before.”

Carter’s announcement caps three years of experimentation at the Pentagon and breakthroughs for women in the armed services. Earlier this year, two female soldiers became the first women to ever graduate from the Army’s grueling Ranger School. But the Pentagon’s project also set off a bitter debate about how women should be integrated.

Carter said that top leaders in the Army, Navy, Air Force and U.S. Special Operations Command all recommended that all jobs be opened to women. The Marine Corps recommended that certain jobs such as machine gunner be kept closed, but the secretary said that the military is a joint force, and his decision will apply to everyone. The top Marine officer who made that recommendation, Gen. Joseph F. Dunford, became chairman of the Joint Chiefs of Staff in September, and did not appear alongside Carter on Thursday.

The services will have 30 days to provide plans to Carter on how they will implement the policy change, he said. By law, the military also must notify Congress formally and wait that long before making any changes.

The roots of the secretary’s decision date back to January 2013, when then-Defense Secretary Leon Panetta announced he was rescinding a longtime ban on women serving directly in ground combat units. Panetta gave the services until this fall to research the issue.

About 220,000 jobs, or about 10 percent, of the military remained closed to women before Thursday’s announcement, Carter said. Another 110,000 jobs in careers like artillery officer were opened in a series of decisions since 2013.

President Obama said in a statement that the Defense Department is “taking another historic step forward” by opening up all positions to women.

“As Commander in Chief, I know that this change, like others before it, will again make our military even stronger. Our armed forces will draw on an even wider pool of talent,” Obama said. “Women who can meet the high standards required will have new opportunities to serve. I know that, under the leadership of Secretary Carter and Chairman Dunford, our men and women in uniform will implement this transition — as they have others — in a responsible manner that maintains military readiness and the unparalleled professionalism and strength of our armed forces.”

The issue has at times opened an uncommonly public rift between senior military leaders. In particular, Navy Secretary Ray Mabus took issue with a Marine Corps study that found that the average woman struggled to keep up with men, according to a number of metrics. The study did not track individual performance, drawing fire from Mabus and others in favor of full integration.

As the Marine Corps commandant, Dunford recommended to keep a number of jobs in infantry and reconnaissance units closed.Carter, asked why Dunford was not present for the announcement on Thursday, said that he and the general have talked extensively on the subject, and he “will be with me” as the services proceeds with making related changes.

“He understands what my decision is, and my decision is my decision, and we will implement it accordingly,” Carter said.

Dunford said in a statement on Thursday afternoon that it is his job to provide his “candid best military advice” to Carter on issues ranging from military readiness, to combat effectiveness, to how the services are employed.

“I have had the opportunity to provide my advice on the issue of full integration of women into the Armed Forces,” Dunford’s statement said. “In the wake of the Secretary’s decision, my responsibility is to ensure his decision is properly implemented. Moving forward my focus is to lead the full integration of women in a manner that maintains our joint warfighting capability, ensures the health and welfare of our people, and optimizes how we leverage talent across the Joint Force.”

Dunford’s spokesman, Navy Capt. Gregory Hicks, said Dunford did not appear Thursday because it was Carter’s decision, and his “opportunity to announce that decision.” Three years ago, Army Gen. Martin E. Dempsey, then the Joint Chiefs chairman, appeared alongside Panetta as he announced that he was rescinding the combat exclusion policy.

Carter said the important factor in him opening all jobs to women was to give the military access to every American who can add strength to it. Studies carried out by the services since 2013 found that some of the standards the military previously used to determine whether a service member was fit for a job were outdated or didn’t reflect the actual tasks required in combat, he said.

“It’s been evidence-based, and iterative,” Carter said of the review. “I’m confident the Defense Department can implement this successfully, because throughout our history we’ve consistently proven ourselves to be a learning organization.”

The Marine Corps will immediately begin the process of implementing the policy change, and share plans and lessons learned with the other services, said Maj. Chris Devine, a Marine spokesman at the Pentagon.

“We are well-informed by our combat experience, as well as our objective approach and data obtained from the past two years of study,” Devine said. “As we move forward with full integration, we’ll continue to maintain our standards, while leveraging every opportunity to optimize individual performance, talent and skills to maximize the warfighting capabilities of our [Marine air ground task forces] in an increasingly complex operating environment.”

Carter cited the military’s 2011 repeal on a policy banning gay service members from serving openly as an example of how gender integration can be completed successfully. The repeal of that “don’t ask, don’t tell” policy raised questions in many corners of the military at the time, but is now widely considered to have been implemented smoothly and without hurting the military’s ability to fight.

The secretary also noted that three women have successfully been able to complete the Army’s Ranger School this year as part of the research into how to better integrate women in the military. The service opened it to women on a full-time basis in September, although the elite 75th Ranger Regiment remained closed to women at the time.

Skeptics remain, however. Sen. John McCain (R-Ariz.) and Rep. Mac Thornberry (R-Tex.), chairmen of the Senate and House armed services committees, said in a statement released jointly on Thursday that they intend to “carefully and thoroughly review all relevant documentation related to today’s decision,” including the Marine Corps gender integration study that caused the rift between the service and Mabus.

“We expect the Department to send over its implementation plans as quickly as possible to ensure our Committees have all the information necessary to conduct proper and rigorous oversight,” the statement said. “We also look forward to receiving the Department’s views on any changes to the Selective Service Act that may be required as a result of this decision.”

Other members of Congress applauded Carter’s decision. Rep. Martha McSally (R-Ariz.), a retired Air Force colonel and A-10 attack jet pilot, said in a statement that the move recognizes that the military is strongest when it prioritizes merit and capability.

“It’s about damn time,” McSally said in the statement. “Women have been fighting and dying for our country since its earliest wars. They have shown they can compete with the best of the best, and succeed. We are a country that looks at people as individuals, not groups. We select the best man for the job, even if it’s a woman.”

Another female combat veteran and member of Congress, Rep. Tammy Duckworth (D-Ill.), released an even more pointed statement of support reflecting her time as an Army helicopter pilot in Iraq and injuries suffered there.

“I didn’t lose my legs in a bar fight — of course women can serve in combat,” she said. “This decision is long overdue.”

Mabus said in a statement that Carter’s decision will maximize the combat effectiveness of the Navy and Marine Corps alike.

“Our process and studies showed that as long as someone can meet operationally relevant, occupation-specific, gender-neutral individual standards, that person is qualified to serve,” Mabus said. “Gender does not define the Service of a United States Sailor or Marine — instead, it is their character, selflessness, and abilities.”

Behind the scenes with the first two women to graduate from Army Ranger School

U.S. Army Soldiers participate in close arm combatives during the Ranger Course on Ft. Benning, GA., April 20, 2015. Capt. Kristen Griest, one of two women becoming the first female soldiers to graduate from Army Ranger School, is at center carrying another soldier and holding a knife. Soldiers attend Ranger school to learn additional leadership and small unit technical and tactical skills in a physically and mentally demanding, combat stimulated environment. (U.S. Army photo by Spc. Nikayla Shodeen/Released Pending Review)

Update: This story has been updated with comments from Navy Capt. Gregory Hicks, a spokesman for Gen. Joseph Dunford.