Boko Haram was founded in 2002 by a charismatic, radical cleric named Mohammed Yusuf, who gained support by speaking out against the rampant corruption and inequality in oil-rich Nigeria.
While Yusuf advocated an extreme Islamist ideology, Boko Haram didn't start using violence as a regular tactic until 2009.
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The turning point came that July when clashes between the group and security forces left 800 people dead, including dozens of police officers.
In an attempt to quell the uprising, Nigerian law enforcement and military personnel carried out the extrajudicial killings of several prominent Boko Haram figures, including Yusuf, according to Human Rights Watch and others.
This allowed a new leader named Abubakar Shekau to take control, and under his rule Boko Haram adopted a strategy of extreme violence.
It has killed Muslims and non-Muslims with impunity, used children as human bombs, burned villages and kidnapped thousands of men, women and children.
The U.S. dispatched drones and aircraft to look for abductees, and has sent personnel to train Nigerian troops. However, President Barack Obama refused to sell aircraft to the country because of human rights concerns.
International watchdogs say the Nigerian military has been guilty of war crimes during its struggle against Boko Haram.
One report by Amnesty International in 2015 detailed gruesome ceremonial killings that involved forcing detainees to dig their own graves and slitting their throats without trial.