Boko Haram: from Islamist sect to armed threat
Nigeria's Boko Haram started out as an Islamic anti-corruption group but mutated into an ISIS affiliate waging a lethal insurgency.
Here is some background about the jihadists, whom Muhammadu Buhari vowed to defeat when he became president in 2015 but remain a threat as he seeks a second term.
Boko Haram aims to create a hardline Islamic state in northeastern Nigeria. Its campaign has cost at least 27 000 lives since 2009 and displaced 1.8 million people.
The name 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 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.
Yusuf was killed in police custody in 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 of 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 opposition to Shekau's indiscriminate targeting of civilians.
Barnawi's faction, which has the backing of ISIS, targets the armed forces and has since July 2018 carried out numerous attacks on military bases.
Shekau's faction is meanwhile responsible for unrelenting suicide bombings targeting civilians.
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