The Nisour Square massacre occurred on September 16, 2007, when employees of Blackwater Security Consulting (now Constellis), a private military company contracted by the US government to provide security services in Iraq, shot at Iraqi civilians, killing 17 and injuring 20 in Nisour Square, Baghdad, while escorting a U.S. embassy convoy. The killings outraged Iraqis and strained relations between Iraq and the United States. In 2014, four Blackwater employees were tried and convicted in U.S. federal court; one of murder, and the other three of manslaughter and firearms charges; all four convicted were pardoned by President Donald Trump in December 2020.Blackwater guards claimed that the convoy was ambushed and that they fired at the attackers in defense of the convoy. The Iraqi government and Iraqi police investigator Faris Saadi Abdul stated that the killings were unprovoked. The next day, Blackwater Worldwide's license to operate in Iraq was temporarily revoked. The U.S. State Department has said that "innocent life was lost", and according to The Washington Post, a military report appeared to corroborate "the Iraqi government's contention that Blackwater was at fault". The Iraqi government vowed to punish Blackwater. The incident sparked at least five investigations, including one from the Federal Bureau of Investigation. The FBI investigation found that, of the 17 Iraqis killed by the guards, at least 14 were shot without cause.

A mercenary, sometimes known as a soldier of fortune or hired gun, is a private individual, particularly a soldier, who takes part in military conflict for personal profit, is otherwise an outsider to the conflict, and is not a member of any other official military. Mercenaries fight for money or other forms of payment rather than for political interests. Beginning in the 20th century, mercenaries have increasingly come to be seen as less entitled to protections by rules of war than non-mercenaries. The Geneva Conventions declare that mercenaries are not recognized as legitimate combatants and do not have to be granted the same legal protections as captured service personnel of a regular army. In practice, whether or not a person is a mercenary may be a matter of degree, as financial and political interests may overlap.