The story is based on Martha Raddatz's New York Times bestseller. On April 4, 2004, the 1st Cavalry Division of the US Army that invaded Iraq was ambushed in Sadr City, Baghdad, and suffered heavy casualties. The US military, still in shock, has since designated this day as the annual "Black Sunday". This incident occurred 11 months after former US President George W. Bush announced that the Iraq War was "mission over". The US military's attitude towards Iraq has changed drastically since then, from so-called "peacekeeping" to massacres. The US military gave it a nice name, called "bravely fighting the insurgents."