NEW : U.S. troop withdrawal rates will depend on ground conditions , Brennan says . The withdrawal comes ahead of President Obama 's August 31 deadline . The drop comes ahead of President Barack Obama 's deadline to bring down troop levels by August 31 . U.S. troops will now switch to an advise - and - assist approach . Vice President Brennan says Iraqi security forces are now successfully protecting civilians .
NEW : U.S. troop withdrawal rates will depend on ground conditions , Brennan says . The withdrawal comes ahead of President Obama 's August 31 deadline . U.S. troops will now switch to an advise - and - assist approach . Vice President Brennan says Iraqi security forces are now successfully protecting civilians .