* Communal massacres, Islamist revolt test Nigeria's unity
* Failed Biafra state was first big rent in the unity
* President insists Nigeria "will not divide"
* But February election risks worsening the strains
By Isaac Abrak, Tim Cocks and Pascal Fletcher
UNGWAN GATA/ABUJA Nigeria, April 28 (Reuters) - When Fulani raiders carrying rifles, machetes and clubs stormed his village one night last month, Pius Nna was stunned to see his teenage nephew among them.
"He was leading them and telling them to check very well, because my house would have a lot of people in it and they would be sure to find someone to kill," said Nna, a tall farmer in his mid-60s who said he escaped by fleeing into the bush.
Sitting in a courtyard littered with rubble, Nna told how his sister's son, whose father is a Muslim Fulani, had led the raiders to burn down his farm in the attack on Ungwan Gata village, one of several mostly Christian Moro'a communities in Nigeria's central Middle Belt.
The March 14 raids by Fulani herders on Ungwan Gata and two other villages killed at least 149 people, locals and officials said. Fulani leaders said their own people had been attacked previously and had a right to defend themselves.
Feuding over land and resources between rival communities in the Middle Belt has killed tens of thousands since independence from Britain in 1960. Fueled by ethnic and religious antagonisms, the violence has been compounded since 2009 in Africa's No. 1 oil producer by a spreading Islamist insurrection in the northeast, led by a group called Boko Haram. That insurgency has killed thousands.
The escalation of conflict, sometimes splitting tribes and families, is straining the divide between Nigeria's largely Muslim north and Christian south and its future as a unified state, recently declared Africa's largest economy.
On April 14, a bus station bombing on the edge of the federal capital Abuja killed at least 75 people. Boko Haram claimed responsibility for the attack, which was followed the same day by a mass abduction of teenage schoolgirls in northeast Borno state, suggesting the state is losing ground against the violence.
A statement following a meeting on Thursday between President Goodluck Jonathan, his security chiefs and all of the country's governors called the surging violence a "war on all Nigerians", urging them to keep religion out of all conflicts.
A Christian southerner whose 2011 election triggered deadly riots, Jonathan may run again in a February 2015 vote which many fear will exacerbate political, ethnic and religious enmities racking Africa's most populous country.
Nigeria, named and created by its British colonial rulers in 1914, was always an artificial formation patching together disparate and often historically antagonistic peoples - principally the largely Muslim Fulani, Hausa and Kanuri of the North, and the Yoruba, Igbo and other peoples of the now mostly Christian south, roughly split across the Middle Belt faultline.
"Nigeria was formed by Britain out of irreconcilable peoples ... these people came to find that, following British rule, the differences among them, far from shrinking, became accentuated, and ... the structure left behind by the British finally was unable to contain the explosive forces contained within it," wrote journalist and author Frederick Forsyth in "The Biafra Story", his 1969 history of the Biafra war.
The 1967-1970 Biafran war, in which the Ibgo easterners tried and failed to form their own breakaway independent state, was the first great rent torn in Nigeria's independence unity.
So far, Boko Haram's northern revolt has not threatened the basis of Nigeria's economic riches, the oil-rich creeks and offshore fields in the south. Niger Delta rebels seeking a greater share of this wealth signed a 2009 peace deal after a campaign of sabotage and violence.
But with violence intensifying and an election coming up in February, there are fears that what Forsyth called Nigeria's "attempted marriage of the irreconcilables" will be tested.
"NIGERIA WILL NOT DIVIDE"
Jonathan says Boko Haram will not "disintegrate" the nation. "Nigeria will not divide," he said in an Easter speech.
In a sign the currents of violence may feed from each other, Boko Haram has invoked lack of justice in Middle Belt killings as justification for its attacks on Christians, including bombings against churches in Plateau and Kaduna states.
Leaders of Christian "indigene" groups say they fear the spread southwards of Muslim Hausa-Fulani is an attempt to impose on them the Islam and sharia law that dominates northern states.
"So, right now, we don't even know whether it's the Boko Haram that is spilling over to us or the indigenous Fulanis that we used to live together with who are coming back to kill our people," Tobias Nkom Maiwada, king of the Attakar people in the Kaura area who have also suffered Fulani raids, told Reuters.
Nigeria's military said on Wednesday "terrorists" - their name for Boko Haram members - were "operating under the guise of herdsmen", but did not offer further explanation.
Boko Haram has targeted villages and schools, Christian churches, car convoys, police, army and government posts and killed hundreds, Muslims and Christians, including civilians.
PRESSURE ON THE LAND
Victims of the March raids in the largely Christian communities of Moro'a farmers identified the attackers as Muslim Fulanis who for years have brought their herds down annually from the drier north to graze in the verdant central pastures.
Local farming communities have also expanded, increasing the pressure on the land, which is studded with hamlets of mud-walled, tin-roofed homes, crop fields and thatched barns.
This contest for land and living space can explode into violence when rival killings ignite old enmities.
Both sides complain inaction by the authorities creates a climate of impunity that feeds reprisal slayings. Since the March 14 attacks, several Fulani have been killed in revenge.
Isah Adamu, a 37-year-old nomadic Fulani, says Fulanis who have settled in local communities often help out the raiders. He cites a Hausa proverb saying "It is only with an insider that a city can be overcome".
Alhaji Haruna Bayero, chairman of the Miyetti Allah Cattle Rearers Association of Nigeria, which seeks to represent Fulani, said many displaced Fulanis felt resentful toward the farmer communities, seeing them enjoying benefits brought by politicians. "Our people felt unsafe and had to leave their homes for other places not very convenient for them," he said.
ATTACK AT NIGHT
Twelve-year-old Joy Likita, who had been sleeping with her sister Afinki in their home in the village of Ungwar Sankwai, woke on the night of March 14 to hear her parents shouting.
Armed men jumped through the window.
"The men were dressed in black," she said. One fired a shot which ricocheted off the roof and hit Joy's mother.
"Then the Fulani man among them who used to live in our village said they shouldn't kill us, but the others refused and shot their guns at us. We all fell to the ground".
"They thought I was dead," said Joy, who received machete wounds to her hand and head. She told her story of survival from her bed in the village medical clinic, tears streaming down her face.
Joy's mother, two sisters and two brothers were killed.
From April 2010 to November 2013, in neighbouring northern Plateau State, similar night attacks on small towns and villages blamed on Fulani raiders killed more than 500 Christians, mostly Berom people, Human Rights Watch said in a December 2013 report.
The report on inter-communal violence in Plateau and Kaduna said since 1992, more than 10,000 people in those two states alone have died in feuding, several thousand since 2010.
ELECTIONS: TRIGGER FOR VIOLENCE
In 2011, Jonathan was elected in a vote which, despite some violence and allegations of fraud, was described by observers as one of the most free and fair ever held in independent Nigeria.
In protest, Hausa-Fulani opposition supporters rioted across northern Nigeria, attacking properties of ruling party officials and Christians and burning churches. In the south of Kaduna state, Christians killed hundreds of Muslims, including Fulanis.
Since Jonathan won a court ruling last year permitting him to run for another term in office, those who oppose him have repeatedly predicted Nigeria would break up if he runs again.
Many northerners felt that in his 2011 election, Jonathan, a former vice-president who took over when northern Muslim Umaru Yar'Adua died in office, tore up an unwritten rule that power should rotate between the north and south every two terms.
Jonathan has not said whether or not he will run, but insists he has a right to. The national conference he convened since last month to thrash out ideas for a possible new constitution seems merely to have highlighted divisions.
"The nature of the people and religious differences are so polarised it's very difficult to find any common ground," Max Siollun, author of Soldiers of Fortune, a history of Nigerian military regimes, told Reuters.
Doubters of the break-up scenario argue that Nigeria, since Biafra, has muddled through intact. They say elites have much to gain from stoking ethnic and religious strife, especially around election time, but everything to lose by pushing it too far.
Competition for oil wealth spurs tension, but a centralised system of distributing revenues to 36 states based on population size is a powerful disincentive for most regions to break away.
Nigeria remains a dynamic entity, especially around Lagos, with large and growing Internet usage, a thriving "Nollywood" entertainment sector and increasing interest from consumer goods firms seeking to tap the huge potential of an economy which was elevated to Africa's largest last month through a GDP rebasing.
"I sometimes joke that Nigeria will stay together at least until the oil runs out," said Siollun.
"After that, all bets are off." (Writing by Pascal Fletcher; editing by Tim Cocks and Janet McBride)