WASHINGTON — After three years of study and debate, Defense Secretary Ash Carter ordered the military Thursday to open all military jobs to women, removing the final barriers that kept women from serving in combat, including the most dangerous and grueling commando posts.

His landmark decision rebuffed requests by the Marine Corps to exclude women from certain infantry and combat jobs and signaled a formal recognition that thousands of women served, and many were wounded or killed, in the last 14 years of war in Iraq and Afghanistan.

“We are a joint force, and I have decided to make a decision which applies to the entire force,” Carter told a news conference.

But he acknowledged some concerns. “Implementation won’t happen overnight. And while at the end of the day this will make us a better and stronger force, there still will be problems to fix and challenges to overcome. We shouldn’t diminish that.”

Carter said the military can no longer afford to exclude half the population from high-risk military posts. He said that any man or woman who meets the standards should be able to serve, and he gave the armed services 30 days to submit plans to make the historic change.

Carter’s order opens the final 10 percent of military positions to women — a total of about 220,000 jobs. And it allows them to serve in the most demanding and difficult jobs, including as special operations forces, such as the Army Delta units and Navy SEALs.

U.S. Rep. Tammy Duckworth, D-Ill., one of the first Army women to fly combat missions in the 2003-2011 Iraq war, welcomed the decision.

“I didn’t lose my legs in a bar fight — of course women can serve in combat,” said Duckworth, whose helicopter was hit by a rocket-propelled grenade. “This decision is long overdue.”

Sen. John McCain, R-Arizona and head of the Senate Armed Services Committee, said Congress will review the data and the decision.

Over the past few years, women have steadily moved into many jobs previously open only to men, including on Navy submarines, in Army artillery units, and as Night Stalkers, the elite special operations helicopter crews, best known for flying the Navy SEALS into Osama bin Laden’s compound in 2011.

Three women became the first to take and pass the Army’s difficult Ranger course.

The military services forwarded their recommendations to Carter earlier this fall. The Army, Navy, Air Force and Special Operations Command all said they would not seek any exceptions and would recommend removing the ban on women in dangerous combat jobs.

Only the Marine Corps sought to keep some jobs closed.

The Joint Chiefs of Staff chairman, Gen. Joseph Dunford, was the Marine Corps commandant at the time and argued that the Marines should be allowed to keep women out of certain front-line combat jobs. He cited studies showing that mixed-gender units aren’t as capable as all-male units.

Months of testing, the Marine review said, found that women often couldn’t carry as much weight or shoot as well as the men. Allowing women to compete for ground combat jobs, it concluded, would make the Marine Corps a less-efficient fighting machine.

Carter on Thursday said he came to a different conclusion, but he said the integration of women into the combat jobs will be deliberate and methodical and will address the Marines’ concerns.

Dunford did not attend the news conference to announce the change, and when pressed about his absence, Carter said he has discussed his decision multiple times with the chairman. In a prepared statement, Dunford said he provided his best military advice on the issue, and now his 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.”

A spokeswoman for the Marines, Maj. Christian Devine, said in a statement that the corps will begin immediately to implement the change, but will maintain the standards of the force while also working to “optimize individual performance.”

Notably, Gen. Joseph Votel, head of U.S. Special Operations Command, said his office also did extensive analysis and decided not to keep any of the high-risk, high-pressure commando jobs closed. Votel said that integrating women into certain jobs in recent years, including in the Special Operations Aviation Regiment and in cultural supports teams in Afghanistan, benefited the force.

“If candidates meet time-tested and scientifically validated standards, and if they have proven that they have the physical, intellectual, professional, and character attributes that are so critical to special operations, they will be welcomed into the special operations forces ranks,” Votel said in a statement.

He and Carter also noted the physical and medical concerns, including data that suggests women are injured more often than men.

“For a variety of reasons, equal opportunity likely will not mean equal participation by men and women in all specialties. There must be no quotas or perception thereof,” said Carter. “The studies that have been done suggest there may be smaller numbers of women in these fields, the fields that were previously closed.”

The services will have to begin putting plans in place by April 1.

Carter has hinted at this decision for months, telling U.S. troops in Sicily in October that limiting his search for qualified military candidates to just half the population would be “crazy.”

Answering a question from a Marine in Sicily, Carter said, “You have to recruit from the American population. Half the American population is female. So I’d be crazy not to be, so to speak, fishing in that pond for qualified servicemembers.”

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Special Operations Command link: https://www.youtube.com/watch?v=JgfmBANgBk8&feature=youtu.be