Defense Secretary Ash Carter on Thursday ordered the military to open all combat jobs to women, rebuffing requests by the Marine Corps to exclude women from certain front-line combat jobs.

Declaring that "we are a joint force," Carter said that while moving women into these jobs will present challenges, the military can no longer afford to exclude half of the population from grueling military jobs.

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, and allows them to serve in the military's most demanding and difficult jobs, including as special operations forces, such as the Army Delta units and Navy SEALs.The Joint Chiefs of Staff chairman, Gen. Joseph Dunford, former Marine Corps commandant, had argued that the Marines should be allowed to keep women out of certain front-line combat jobs, citing studies showing that mixed-gender units aren't as capable as all-male units.

Carter 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 Marine Corps concerns.Dunford did not attend the news conference to announce the change, and when asked about that 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.

"While noting that, on average, men and women have different physical abilities, Carter said the services must assign tasks and jobs based on ability, rather than on gender. He said that would likely result in smaller numbers of women in some jobs. Equal opportunity, he said, will not mean equal participation in some specialty jobs. But he added that combat effectiveness is still the main goal, and there will be no quotas for women in any posts.

The decision comes after several years of study, and will wipe away generations of limits on how and where women can fight for their country.Only the Marine Corps sought any exceptions in removing the long-held ban on allowing women to serve in dangerous combat jobs. The Army, Navy and Air Force have moved steadily toward allowing women to serve in all posts, and only the most risky jobs remain closed.

A senior defense official said the services will have to begin putting plans in place by April 1. The official was not authorized to discuss the matter publicly and spoke on condition of anonymity.

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.

"He had given Dunford until the end of October to forward his review of the services' recommendations on which jobs, if any, should remain closed to women. As Marine commandant, Dunford was the only service chief to recommend that some front-line combat jobs stay male-only, according to several U.S. officials.

Carter had pledged to thoroughly review the recommendations, particularly those of the Marine Corps, but said he generally believes that any qualified candidate should be allowed to compete for jobs.

But the senior defense official said that while Carter recognizes there may be difficulties in opening the jobs to women, he has made his decision and all the services will follow it.

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 service members."For that reason, the defense secretary said the military should recruit women into as many specialties as possible.