The U.S. Defence Department’s decision to lift the ban on women serving in combat presents a daunting challenge to military leaders who will have to decide which jobs they believe should be open only to men.

Defence Secretary Leon Panetta announced on Thursday more than 230,000 battlefront posts, many in Army and Marine infantry units and in potentially elite commando jobs, are now open to women. The historic change, recommended by the Joint Chiefs of Staff, overturns a 1994 rule prohibiting women from being assigned to smaller ground combat units.

Some jobs may open as soon as this year, while assessments for others, such as special operations forces, may take longer. The services will have until January 2016 to make a case to that some positions should remain closed to women, said a senior military official.

As news of Mr. Panetta’s order got out, many members of Congress, including the Senate Armed Services Committee chairman, Democratic Senator Carl Levin, announced their support.

Objections were few. Jerry Boykin, executive vice president of the Family Research Council, called the move “another social experiment” that will place unnecessary burdens on military commanders.

He noted that small units often are in sustained combat for extended periods of time under primal living conditions with no privacy.

Mr. Panetta’s move comes in his final weeks as Pentagon chief and just days after President Barack Obama’s inaugural speech in which he spoke passionately about equal rights for all.

The new order expands the department’s action of nearly a year ago to open about 14,500 combat positions to women, nearly all of them in the Army.

In addition to questions of strength and performance, there also have been suggestions that the American public would not tolerate large numbers of women being killed in war.

Under the 1994 Pentagon policy, women were prohibited from being assigned to ground combat units below the brigade level.

A brigade is roughly 3,500 troops split into several battalions of about 800 soldiers each.

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