Why they are sending 11,000 troops

"But in purely military terms, sending 11,000 U.S. and Afghan troops to defeat a few hundred Taliban fighters in Marja won't change much in Afghanistan. The greater significance of the battle is in how it is perceived in the rest of Afghanistan and in America. The campaign's goals are to convince Americans that a new era has arrived in the eight-year-long war and to show Afghans that U.S. forces and the Afghan government can protect them from the Taliban.
...Military officials in Afghanistan hope a large and loud victory in Marja will convince the American public that they deserve more time to demonstrate that extra troops and new tactics can yield better results on the battlefield." http://www.washingtonpost.com/wp-dyn/content/article/2010/02/21/AR2010022104201.html