Murray, who leads the Senate Veterans' Affairs Committee and is a member of the Defense Appropriations Subcommittee, called Panetta's decision a "historic step for equality" that recognizes the role women play in the military.

The Pentagon must notify Congress of each job or unit as it is sent up to the secretary to be opened to women. Then the Defense Department must wait 30 days while Congress is in session before implementing the change.

It is a marked difference from the way the military ended the exclusion of gays serving openly, or the "don't ask, don't tell" policy. In that case, there were no stipulations attached to openly gay service members. There was no staggered approach that integrated openly gay troops into units. It was instead done all at once, across the board.

A senior defense official explained the Pentagon's reasoning behind the different approach: "You're talking about personal choice of behavior versus physical capability. And they were already in the units. If you take a unit that's never had women before, that's quite a culture change."

Another senior defense official said the goal is "to provide a level, gender-neutral playing field."

The American Civil Liberties Union recently filed a federal lawsuit against the Department of Defense, charging that combat exclusion is unfair and outdated, harms America's safety and prevents women from receiving training and recognition for their work. The plaintiffs, who include women awarded Purple Hearts, say the exclusion places them at a disadvantage for promotion.

The ACLU said it is thrilled about Panetta's planned announcement.

"But we welcome this statement with cautious optimism, as we hope that it will be implemented fairly and quickly so that servicewomen can receive the same recognition for their service as their male counterparts," Ariela Migdal, senior staff attorney with the ACLU Women's Rights Project, said in the statement.

Earlier this month, the Army opened the 160th Special Operations Aviation Regiment to women, and it has begun recruiting female pilots and crew chiefs. The Navy has put its first female officers on submarines in the past year, and certain female ground troops have been attached to combat units in Iraq and Afghanistan. More than 800 women were wounded in those wars, and at least 130 have died.