US Defence Secretary Ash Carter has announced that he is ordering the American military to open all combat jobs to women, rebuffing requests by the Marine Corps to exclude women from certain frontline roles.
Declaring that "we are a joint force", Mr 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 gruelling military positions.
He said 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.
Mr Carter's order opens the final 10% of military roles to women, and 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.
The Joint Chiefs of Staff chairman, General Joseph Dunford, former Marine Corps commandant, had argued that the Marines should be allowed to keep women out of certain frontline combat jobs, citing studies showing that mixed-gender units are not as capable as all-male units.
Mr Carter said he came to a different conclusion, but added that the integration of women into the combat roles will be deliberate and methodical and will address the Marine Corps' concerns.
Gen Dunford did not attend the news conference to announce the change, and when asked about that absence, Mr Carter said he has discussed his decision multiple times with the chairman.
While noting that, on average, men and women have different physical abilities, Mr Carter said the services must assign tasks and jobs based on ability, rather than on gender.
He said that was likely to result in smaller numbers of women in some jobs. Equal opportunity, he said, will not mean equal participation in some speciality roles. 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 towards allowing women to serve in all posts, and only the most risky jobs remain closed.
A senior defence official said the services will have to begin putting plans in place by April 1.
Mr Carter has hinted at the decision for months, telling US troops in Sicily in October that limiting his search for qualified military candidates to just half the population would be "crazy".
He had given Gen 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, Gen Dunford was the only service chief to recommend that some frontline combat jobs stay male-only, according to several US officials.
Mr 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 defence official said that while Mr Carter recognises there may be difficulties in opening the roles to women, he has made his decision and all the services will follow it.
Answering a question from a Marine in Sicily, Mr 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 defence secretary said the military should recruit women into as many specialities as possible.