Boko Haram started out as an Islamist group railing against corrupt officials in northeast Nigeria, but its decade-long insurgency has now devastated the region.
The campaign of violence has cost at least 27,000 lives, displaced some two million people and spawned an affiliate of the Islamic State group. But what is Boko Haram.
Hardline Islamic state
Boko Haram aims to create a hardline Islamic state in northeastern Nigeria.
The group's nickname loosely translates from the Hausa language as "Western education is forbidden".
Its founder and spiritual leader Muhammad Yusuf pinned the blame for Nigeria's ills on Western values left by former colonial master Britain.
He also accused the country's secular leaders of corruption and neglecting development in Muslim regions.
Yusuf came to the attention of authorities in 2002 when he began to build a following among disaffected youths in Maiduguri, the Borno state capital in northeast Nigeria.
He was killed in police custody in July 2009 after an uprising in Maiduguri that prompted a military assault. Some 800 people died in the action, and Boko Haram's mosque and headquarters were left in ruins.
Many of its supporters fled the country.
Boko Haram was broadly peaceful before Yusuf's death.
But his successor, his right-hand man Abubakar Shekau, undertook a violent campaign of deadly attacks on schools, churches, mosques, state entities and security forces.
Some Boko Haram members are thought to have trained with Al-Qaeda in the Islamic Maghreb (AQIM) in northern Mali in 2012 and 2013.
Among the group's most notorious acts was the April 2014 kidnapping of 276 schoolgirls from the remote northern town of Chibok in Borno state.
Fifty-seven fled in the immediate aftermath, and more than 100 of the 219 who spent years in captivity have since been released, found or escaped.
The mass abduction brought world attention to the insurgency at a time when Boko Haram was seizing territory across the northeast, which became a largely no-go area, with the violence spilling over into Cameroon, Chad and Niger.
In August 2014 Shekau proclaimed a "caliphate" in the Borno town of Gwoza, and in March 2015 pledged allegiance to the Islamic State group in Iraq and Syria.
The violence has destroyed property and farmland in the mainly rural northeast and sparked a humanitarian crisis and acute food shortages.
An offensive since 2015 by regional armies -- troops from Nigeria and backed by others from Cameroon, Chad and Niger -- drove jihadists from most of the areas they had seized.
But regular bloody raids and suicide bomb attacks continue.
Mass court action for people suspected of being members of Boko Haram started in October 2017. Most were released, largely due to lack of evidence, and more than 100 convicted for belonging to the group and taking part in attacks.
A faction led by the son of Muhammad Yusuf, Abu Mus'ab al-Barnawi, broke away in 2016 in opposition to Shekau's indiscriminate targeting of civilians.
The splinter Islamic State West Africa Province (ISWAP) group has the backing of IS. It targets the armed forces and has since July 2018 carried out numerous attacks on military bases.
Shekau's faction is meanwhile responsible for suicide bombings targeting civilians.
Since last year Barnawi has reportedly been sidelined in ISWAP by more radical commanders.