On Dec 15, 2011 the last combat troops that had taken part in Operation Iraqi Freedom (OIF) and the transitional Operation New Dawn (OND) left Iraq. It came over seven years after OIF began with the March 2003 invasion to remove Saddam Hussein. Around 4,500 U.S. troops died during that phase of American military involvement.
The number of fatalities is picking up again ever since the United States once more got involved in Iraq in August 2014, this time to help Iraqi forces combat the so-called Islamic State (IS). This engagement is called Operation Inherent Resolve (OIR). Further official data on U.S. casualties can be retrieved here.