In a historic shift in policy, the Pentagon will open all combat jobs to women, Defense Secretary Ash Carter announced Thursday.

"Our force of the future must continue to benefit from the best America has to offer," Carter told reporters during a Pentagon press conference. "This includes women."

Women will now be able to help fill some of the 220,000 roles that are currently only open to their male counterparts — positions that include some special operations units and infantry — among other roles.

The policy change will take effect in 30 days, 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 said. "They'll be able to drive tanks, give orders, lead infantry soldiers into combat."

Related: Military to Mull Opening all Combat Jobs to Women or Get Waivers

The move comes about two years after former Defense Secretary Leon Panetta announced plans to change the rule excluding women from serving in combat roles.

In late September, the military services had to submit recommendations about opening all combat jobs to women — deciding whether they will open everything up or if they will request waivers to continue to prohibit women from serving in certain areas.

The Navy and Air Force had already made it clear that they will open all positions to women. The Army had not said what they would do at that point, but women being allowed to go through Ranger School was one of the first steps in that direction.

Unlike the other services, when Panetta rescinded the Direct Ground Combat Rule in January 2013, a rule that prohibited women from serving in combat jobs, the Marines began a nearly year-long, multi-million dollar research study to determine whether female Marines could serve in combat roles.

Related: Two Women Make History by Passing Army's Elite Ranger School

The Marines requested an exception to prohibit women from serving in certain areas and provided data and surveys to back up their concerns about the ability to perform and a higher number of injuries.

The $36 million study included about 100 women and about 300 men — all volunteers.

A U.S. military official provided some of the results of that study today. Among the results:

Women were injured twice as often as the men (40.5% for women, 18.8% for men)

Men were more accurate at shooting on every weapon system except the M4

Women had trouble with combat tasks, including removing casualties

U.S. military officials had previously said the study provided empirical evidence that certain combat roles — specifically artillery and infantry — may be too physically demanding for women.

On Thursday, Carter made clear that "there will be no exceptions" to allowing women capable of fulfilling duties from serving in all combat roles.

"I came to a different conclusion," Carter said of the data provided by the Marines.

Carter added that the key would be implementation and cautioned that the changes would take time.

So will changing attitudes within the military.

"Some service members, men and women, have a perception that integration would be pursued at a cost of combat effectiveness," Carter said underscoring that women seeking to fill the new roles open to them would be held to the same physical requirements.

However, women make up more than half of the nation's population and should be given an equitable chance to serve in all military combat roles, Carter said.

"The military has long prided itself on being a meritocracy," he said.