BAGHDAD | A once-feared Shiite militia that was crippled two years ago by defections and a U.S.-Iraqi crackdown has quietly started to regroup, adding street muscle to the Shiite party that emerged strongest from Iraq's parliamentary elections. The revival of the Mahdi Army, loyal to radical cleric Muqtada al-Sadr, could be an ominous sign. An al-Sadr spokesman says the force is gearing up to ensure U.S. forces stick to a Dec. 31, 2011 deadline to withdraw from the country — threatening attacks on American troops if they stay past the date. In the near-term, Sunnis fear the militia will turn...