On September 11, 2012, the United States mission in Benghazi, Libya was attacked, causing the death of four Americans, including US ambassador Christopher Stevens. The attack was initially thought to be a spontaneous protest due to an anti-Islam video, but it was determined that the assault was a planned terrorist act. An independent review of the attack in December 2012 reported that there were "systemic failures and leadership and management deficiencies" in the US State Department and another report by the Senate Intelligence Committee in January 2014 stated that the attack was "likely preventable." However, in November 2014 the House Permanent Select Committee on Intelligence reported that there was no intelligence failure prior to the attack and no stand-down order to CIA operatives trying to assist the consular building. They also reported conflicting intelligence in the wake of the attack about the motive and cause (source).
Hillary Clinton was Secretary of State during the Benghazi attack, so her administration and handling of the situation has been highly scrutinized in the attempt to determine if the attack was preventable, and if so, who is at fault. On October 22, Clinton publicly testified before the House Select Committee.
The question of whether Clinton is responsible for allowing the attacks to happen has become an increasingly partisan issue, with Republicans more likely to be critical of Clinton's response and Democrats more likely to believe that she did not mishandle the attack. Many Democrats were also critical for the need to further investigate the Benghazi attack. Also, given Clinton's current run for President, many Republicans hoped to use the Oct. 22 hearing to damage her campaign. However, many people were more impressed by Clinton's calm demeanor than by the mainly Republican line of questioning from the Committee (source).
The hearing did reveal that Clinton had told an Egyptian official and her family that the attack was done by a terrorist group, despite the current statements saying that the attack was due to the anti-Islam video. Clinton maintained that the conflicting reports were due to the confusing flood of information coming into the State Department and the "fog of war" (source).
Pictures: US mission in Benghazi in flames the day of the attack(source), Clinton during the hearing (source).