WASHINGTON—Defense Secretary Robert Gates has decided to send an additional 1,400 Marine combat forces to Afghanistan, officials said, in a surprise move ahead of the spring fighting season to try to cement tentative security gains before White House-mandated troop reductions begin in July.

The Marine battalion could start arriving on the ground as early as mid-January. The forces would mostly be deployed in the south, around Kandahar, where the U.S. has concentrated troops over the past several months.

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