( CNN ) -- The number of U.S. troops in Iraq has fallen below 50,000 -- the lowest level since the U.S-led invasion in 2003 , the military said Tuesday . The drop comes ahead of President Barack Obama 's deadline to bring down troop levels by August 31 . The military said U.S. troops will now transition to Operation New Dawn , effective September 1 . In this phase , the United States will help train Iraqi security forces until the end of its mission in December 2011 . While the total number of U.S. troops in Iraq will continue to diminish throughout 2011 , the withdrawal rate will be `` calibrated and adjusted based on conditions on the ground , '' according to presidential national security adviser John Brennan . On Monday , Vice President Joe Biden said that the overall level of violence in Iraq has now declined to the point that some early U.S. veterans of the conflict `` would not recognize '' the country today . The roughly 650,000-member Iraq security force is `` already leading the way to defend and protect [ the ] country , '' he said . `` Some said that our drawdown would bring about more violence . Well , they were wrong , '' Biden said at a gathering of the Veterans of Foreign Wars in Indianapolis , Indiana . Iraqi officials , however , have noted a recent campaign of bombings and shootings in Baghdad , where gunmen have attacked traffic police and checkpoints , killing Iraqi troops . Many Iraqis have blamed the recent violence on the current political paralysis , in which quarreling parties have failed to form a government nearly six months after parliamentary elections . The Iraqi government had been pushing to increase security ahead of the U.S. drawdown . Biden stressed that the changing U.S. role in Iraq should be not be viewed as American disengagement . He noted that the administration is largely following former President George W. Bush 's plan for a `` long-term '' relationship with Baghdad . Civilian efforts to promote Iraqi stability are ramping up , Biden said . But Obama is planning a speech on the situation in Iraq after he returns to Washington from vacation on August 29 , a senior administration source said Sunday .
NEW : U.S. troop withdrawal rates will depend on ground conditions , Brennan says . The drop comes ahead of President Obama 's Monday deadline . Iraq troops will now switch to an advice-and-assist role . Vice President Obama says American security forces are now successfully protecting U.S. .