Why are we asking this now?
Upto 265 people are reported to have died in the Nigerian city of Jos after fighting between Muslims and Christians. Calm has now been restored but only after a 24-hour curfew imposed by the government which has sent soldiers armed with machine guns to patrol the streets in pick-up trucks. But there are reports that the violence has now spread to Pankshin, 60 miles to the south-east.
By an unwritten agreement Nigeria's ruling party alternates between a Christian and Muslim leader. The current president, Umara Yar'Adua, is a northern Muslim, but he has not been seen in public since November 23. Indeed for the last 50 days he has been in hospital in Saudi Arabia with heart and kidney problems.
His vice-president, the felicitously-named Goodluck Jonathan, is a southern Christian. If he takes over there may be Muslim resentment that their "turn" in office has been foreshortened.
Have there been religious riots here before?
Jos is in the volatile middle belt of Nigeria. With more than 140 million people and 250 ethnic groups, Nigeria divides between a largely Muslim north and a Christian and animist south. Jos is on the ethnic and political faultline between the two areas. Violent sectarian eruptions are not uncommon.
As many as 1,000 were killed in Jos in unrest in 2001. Another 500 people were killed in 2004 when violence broke out between Muslim Hausas and Christian Beroms in another northern town, Yelwa. Then 700 died in a similar gruesome outbreak of killing in Jos in 2008. Various state and federal commissions of inquiry were set up but they have yet to report. Christian campaigners have complained that no-one has been prosecuted for their roles in the previous clashes.
What's the cause of the trouble?
No one really knows. Some said the weekend violence flared after a football match. Others say Christians were attacked in their churches. Others that it grew from attacks on a Muslim repairing his house which had been damaged in the 2008 violence.But many leaders of both religions insist that the conflicts are not religious at all but rooted in tensions created by poverty.
Nigeria is one of the richest nations in Africa, as well as one of the most populous. But most of its huge population see none of its massive oil wealth. Nigeria is run by an elite of millionaires so corrupt and ineffective they do not deliver even basic services like running water and electricity to the people. Violence often grows from disputes over access to resources such as land.
Where does the President come in?
Umara Yar'Adua came to office in 2007 promising big changes – tackling corruption, shaking up the inadequate power sector and reforming the flawed electoral system. But three years on he was known by the nickname "Baba-go-slow".
The only area in which he has made real progress is in personally persuading militants in the oil-rich Niger Delta [they are demanding that the region gets a greater share of the oil wealth] to agree to a three-month amnesty. And now even that is threatening to unravel because of the power vacuum that is growing around the absent President.
Why doesn't the vice-president take over?
Under Nigeria's constitution, the vice-president has few real powers. To take over he needs the President to delegate temporary authority to him. But Mr Yar'Adua has pointedly not done this. In a country where politics is a perpetual power struggle within the wealthy elite, the Nigerian cabinet – which could force a transfer of interim powers – has declined.
What will follow from this political paralysis?
Critical reforms to the oil industry and the banking sector are falling apart. The 2010 budget cannot go through parliament without action by the President. There is no one to swear in the country's Chief Justice, so Nigeria has a judiciary without a head as well as an executive without a president. There is no progress on the endemic and monumental problem of corruption, though the head of the anti-corruption agency, Nuhu Ribadu, had already been sidelined under Mr Yar'Adua. The amnesty with militants in the Niger Delta looks like it is derailing.
Mr Yar'Adua tried to quell rumours about his incapacity – or even death – by giving a telephone interview to the BBC last week from his Saudi hospital bed. But he sounded so frail it only accelerated the plotting and manoeuvring back home.
Nigeria's retired generals – the country was a military dictatorship until just 10 years ago – and elder statesmen like the former heads of state Olusegun Obasanjo and Ibrahim Babangida are said to be jockeying for position and pushing favoured candidates to replace Mr Yar'Adua.
Is all this affecting the economy?
Undoubtedly. Violence in the Niger Delta – where companies such as Shell and Exxon pump the oil that makes Nigeria the fifth-biggest source of US oil imports – had reduced Nigeria's crude output by a quarter over the past three years. Mr Yar'Adua's amnesty had sharply reduced the armed attacks that had curbed production.
But last week four foreign contractors working for Shell – three Britons and a Colombian – were seized by armed kidnappers. It was the first abduction of foreigners since July, compared with an average of one a week at the peak in 2007, raising fears that the amnesty is over now that President Yar'Adua is off the scene. More generally investors are delaying investment decisions in Africa's second-biggest economy.
What happens now?
The Chief Justice of the federal High Court in Abuja, Dan Abutu, has said he will rule tomorrow on a lawsuit seeking to declare Mr Yar'Adua incapable of performing his duties. That would pave the way for Goodluck Jonathan to take over.
All this matters because Nigeria is, along with South Africa, the potential economic motor for the whole of Africa, and a potential force for peace. But adrift it becomes a threat to stability throughout the entire west of the continent.
Having the right people in charge is key. That was shown when Mrs Ngozi Okonjo-Iweala was Nigeria's finance minister from 2003 to 2006. She introduced a raft of reforms and things improved. Nigeria went from the most corrupt nation on earth to just the sixth most. It was major progress for just three years. But the corrupt elite got rid of her and things slipped backwards.
What Nigeria needs is a commitment to openness about government revenues and budget allocations. It needs the industrialised world to prosecute Western firms which pay big bribes there. It needs internationally agreed codes and standards. It needs transparency in the procurement of government contracts and much more. But it will get none of that without a strong leader, which at present it clearly does not have.
Could religion and politics tear Nigeria apart?
* Sectarian conflict is endemic on the faultline between the Muslim north and the Christian and animist south
* Politics is not about policy but a perpetual power struggle within the wealthy elite
* Whenever a real reformer comes along to tackle corruption the self-serving elite gets rid of them
* The true problem is poverty (not religion) and corruption – and only politics can change that
* Umara Yar'Adua has shown a peace deal with the militants in the oil-rich Niger Delta is a political possibility
* Nigeria can be the economic engine for Africa, and a force for peace and stability – if it has a strong leader