Boko Haram has been one of the world's deadliest militant groups over the past decade, kidnapping and killing tens of thousands of civilians across northeast Nigeria and beyond.
The Islamic State-linked sect, whose name roughly translates to "Western education is a sin," achieved worldwide infamy after it kidnapped 274 female high school students from the town of Chibok in April 2014.
This spurred the #BringBackOurGirls campaign — backed by Michelle Obama, Ellen Degeneres, Dwayne Johnson and other celebrities.
Some of those girls were released years later, but thousands of other abductees who didn't make the headlines remain missing.
Many of those who weren't enslaved as "wives" and raped are believed to have been used as human bombs — strapped with explosives and sent into crowded markets to cause carnage.
Boko Haram has been involved in some 30,000 killings over the past decade, according to figures from the Council on Foreign Relations, a Washington-based think tank.
How did Boko Haram start?
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.
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.
Boko Haram was deemed the world's deadliest terror group in 2015 — killing more people than the Islamic State that year.
What has the U.S. done?
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.
In January 2017, the Nigerian Air Force killed more than 100 civilians after it mistakenly bombed a refugee camp while trying to target the group.
President Donald Trump later agreed to sell 12 Super Tucano A-29 planes and weapons to Nigeria in a deal worth $593 million.
What about the Nigerian government?
The government in Abuja has claimed to have killed Shekau three times only for the group's leader to resurface alive and well in later videos.
While Boko Haram attacks continue, the level of violence has decreased under Muhammadu Buhari, a former dictator who was elected Nigeria's president in 2015.
However, the group still killed around 1,000 people in the 12 months from May 2017 and this May, according to the Council on Foreign Relations.
Boko Haram has also splintered into several factions. One, known as the Islamic State in West Africa, or ISWAP, has allied itself with ISIS in the Middle East.
Unlike Shekau's faction, ISWAP is believed to distinguish itself by refusing to kill Muslims.
A Reuters report in April said ISWAP was administering an area of land 100 miles long, creating a taxation economy designed to win over local residents.