Here is a factfile on Boko Haram and its rival ISWAP following the killing of 24 Chadian troops in the troubled Lake Chad area.
The insurgency initiated by Boko Haram in 2009 has left more than 36,000 dead, mainly in Nigeria, while three million people across the region have been forced to flee their homes, according to UN estimates.
The Chadian authorities said the troops "were attacked by Boko Haram" on Wednesday, but officials also use the term to refer to the group's bitter rivals, the Islamic State West Africa Province (ISWAP).
Boko Haram launched its campaign with the goal of creating a hardline Islamic state in Nigeria.
The group's name is Jama'tu Ahlis Sunna Lidda'awati wal-Jihad (JAS) but it is commonly called Boko Haram, which loosely translates from the Hausa language as "Western education is forbidden".
Its founder and spiritual leader Mohammed Yusuf blamed Nigeria's ills on western values left by former colonial master Britain. He also accused the country's secular leaders of corruption and neglecting northern Muslim regions.
Yusuf built a following among disaffected youths.
He was killed in police custody in July 2009 after an uprising in Maiduguri, Borno state, that prompted a fierce 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.
Descent into killing
After Yusuf died his successor, Abubakar Shekau, undertook a violent campaign of attacks on schools, churches, mosques and state 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 town of Chibok.
The mass abduction brought worldwide notoriety and came as Boko Haram was turning swathes of northeast Nigeria into a no-go area.
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 sparked a humanitarian crisis and acute food shortages in the mainly rural northeast.
An offensive launched in 2015 by Nigerian troops backed by soldiers from Cameroon, Chad and Niger drove jihadists from most of the area.
Nigerian President Muhammadu Buhari declared in December that year that Boko Haram was "technically" defeated.
But the group and its dissident offshoot, ISWAP, still staged deadly attacks on both military targets and civilians.
Mass trials of suspects started in 2017 but most were released due to lack of evidence, with some 100 locked up.
Split and resurgence
Boko Haram splintered in 2016 over disagreements about Shekau's indiscriminate targeting of Muslim civilians and use of children and women as suicide bombers.
The dissenting faction ISWAP, originally established by Muhammad Yusuf's son Abu Mus'ab al-Barnawi, has the backing of the Islamic State group.
ISWAP is particularly active on the Chad and Niger borders and has since 2018 carried out numerous attacks on military bases.
The group is also behind the abduction and killing of Christians and aid workers.
Boko Haram claimed in 2020 to have been responsible for abducting hundreds of young boys from a school in Kankara in the northwest, far from its Sambisa forest base. They were released after a week.
Over the past few months mass abductions have increased in Nigeria, mostly carried out by criminal groups for ransom, some of which are believed to have forged links with jihadists.
Boko Haram confirmed in mid-June 2021 that Shekau had been killed in fighting with ISWAP.