The groundbreaking decision overturns a 1994 Pentagon rule that restricts women from artillery, armor, infantry and other such combat roles, even though in reality women have frequently found themselves in combat in Iraq and Afghanistan, where more than 20,000 have served. As of last year, more than 800 women had been wounded in the two wars and more than 130 had died.
Defense officials offered few details about Mr. Panetta’s decision but described it as the beginning of a process to allow the branches of the military to put the change into effect. Defense officials said Mr. Panetta had made the decision on the recommendation of the Joint Chiefs of Staff.
Women have long chafed under the combat restrictions and have increasingly pressured the Pentagon to catch up with the reality on the battlefield. The move comes as Mr. Panetta is about to step down from his post and would leave him with a major legacy after only 18 months in the job.
The decision clearly fits into the broad and ambitious liberal agenda, especially around matters of equal opportunity, that President Obama laid out this week in his Inaugural Address. But while it had to have been approved by him, and does not require action by Congress, it appeared Wednesday that it was in large part driven by the military itself. Some midlevel White House staff members were caught by surprise by the decision, indicating that it had not gone through an extensive review there.
Mr. Panetta’s decision came after he received a Jan. 9 letter from Gen. Martin E. Dempsey, the chairman of the Joint Chiefs of Staff, who stated in strong terms that the armed service chiefs all agreed that “the time has come to rescind the direct combat exclusion rule for women and to eliminate all unnecessary gender-based barriers to service.”
A military official said the change would be implemented “as quickly as possible,” although the Pentagon is allowing three years, until January 2016, for final decisions from the services.
Each branch of the military will have to come up with an implementation plan in the next several months, the official said. If a branch of the military decides that a specific job should not be opened to a woman, representatives of that branch will have to ask the defense secretary for an exception.
“To implement these initiatives successfully and without sacrificing our war-fighting capability or the trust of the American people, we will need time to get it right,” General Dempsey wrote.
It will be carried out during what the administration describes as the end of the American combat role in Afghanistan, the nation’s longest war.
A copy of General Dempsey’s letter was provided by a Pentagon official under the condition of anonymity.
The letter noted that this action was meant to ensure that women as well as men “are given the opportunity to succeed.”
It was unclear why the Joint Chiefs acted now after examining the issue for years, although in recent months there has been building pressure from high-profile lawsuits.
In November 2012 the American Civil Liberties Union filed a federal lawsuit challenging the ban on behalf of four service women and the Service Women’s Action Network, a group that works for equality in the military. The A.C.L.U. said that one of the plaintiffs, Maj. Mary Jennings Hegar, an Air National Guard helicopter pilot, was shot down, returned fire and was wounded while on the ground in Afghanistan, but could not seek combat leadership positions because the Defense Department did not officially acknowledge her experience as combat.
In the military, serving in combat positions like the infantry remains crucial to career advancement. Women have long said that by not recognizing their real service, the military has unfairly held them back.
The A.C.L.U. embraced Mr. Panetta’s decision with cautious optimism. Ariela Migdal, an attorney with the A.C.L.U.’s Women’s Rights Project, said in a statement that the organization was “thrilled” by the decision, but added that she hoped it would be implemented “fairly and quickly.”
By law Mr. Panetta is able to lift the ban as a regulatory decision, although he must give Congress a 30-day notice of his intent. Congress does not need to approve the decision before it goes into effect. If Congress disagrees with the action, members would have to pass new legislation prohibiting the change, which appeared highly unlikely.
Although in the past some Republican members of the House have balked at allowing women in combat, on Wednesday there appeared to be bipartisan endorsement for the decision, which was first reported by The Associated Press and CNN in midafternoon.
“It reflects the reality of 21st century military operations,” Senator Carl Levin, Democrat of Michigan and chairman of the Senate Armed Services Committee, said in a statement.
SenatorPatty Murray, Democrat of Washington and the chairwoman of the Senate Veterans Affairs Committee, called it a “historic step for recognizing the role women have, and will continue to play, in the defense of our nation.”
Senator Kelly Ayotte, a New Hampshire Republican and a member of the Armed Services Committee, said in a statement that she was pleased by the decision and said that it “reflects the increasing role that female service members play in securing our country.”
Representative Loretta Sanchez, the California Democrat who has long pressed to have women’s role in combat recognized, said that she was pleased that Mr. Panetta was removing what she called “the archaic combat exclusion policy.”
SenatorKirsten E. Gillibrand, a New York Democrat who has pushed for lifting the ban, called it “a proud day for our country” and an important step in recognizing “the brave women who are already fighting and dying.”
But the leadership of a conservative Christian group, the Family Research Council, immediately weighed in with its opposition, sending out a statement from Jerry Boykin, a retired three-star general with a long career in Special Operations Forces.
General Boykin said that “the people making this decision are doing so as part of another social experiment.” He especially criticized the concept of placing women into Special Forces units where “living conditions are primal in many situations with no privacy for personal hygiene or normal functions.” It remains unclear if women will be permitted to fight in Special Forces and other commando units.
Public opinion polls show that Americans generally agree with lifting the ban. A nationwide Quinnipiac University poll conducted a year ago found that three-quarters of voters surveyed favored allowing military women to serve in units that engaged in close combat, if the women wanted to.
Policy experts who have pushed the military to lift the ban said that it was striking that much of the impetus appeared to come from Joint Chiefs, indicating that the top military leadership saw that the time had come to open up to women.
“It’s significant that the change came from the uniformed side, rather than being forced on the uniformed side by the civilian leadership,” said Greg Jacob, the policy director of the Service Women’s Action Network.
Under current rules, a number of military positions are closed to women — and to open them, the services have to change the rules.
Under Mr. Panetta’s new initiative, the situation is the opposite: Those combat positions would be open to women, and they could only be closed through specific action.
Capt. Emily Naslund, a Marine officer who saw ground combat in Afghanistan in 2010, said Wednesday that she embraced the decision. “This is awesome,” she said.