By David Loyn
BBC World Affairs correspondent
If the UK and the US thought things were bad in Iraq, they suddenly got worse with the sight of Grand Ayatollah Ali Sistani arriving in London for medical treatment.
The ayatollah represents the conservative mainstream of Iraqi Shias
The 73-year-old is the most senior religious leader of Iraq's majority Shia community. He is the man credited with the truce which ended the most bitter fighting in Najaf two months ago.
But it is misleading to call Ayatollah Sistani a moderate.
He is as keen as any Iraqi for US forces to leave the country.
The only difference is over tactics.
His main contribution to stability has been to oppose all violence, and to try to negotiate a peaceful resolution of Iraq's problems.
That has meant that the Shias have, by and large, kept out of the wider revolt against the US in the so-called Sunni triangle in the west, and in Baghdad itself.
But with Ayatollah Sistani out of the way in London - and sick enough for there to be talk of succession - the strength of the younger Shia leader, Moqtada Sadr, is now being tested for the first time.
The fighting broke out after Iraqi government forces, backed by the US, attacked Mr Sadr's Mehdi army in the large Muslim cemetery in Najaf which they have made their base.
Sadr is the son of one of Iraq's most revered Shia leaders
It is still disputed whether they were provoked first.
Despite significant losses on the insurgent side and the exact scale still unclear, Mr Sadr's spokesman said: "This is a revolution against the occupation force until we get independence and democracy".
A message read at Friday prayers in Kufa, where Mr Sadr lives, called the US "the enemy".
Whoever started the fighting, it will be difficult to stop.
Heavy-handed US military tactics continue to alienate young Iraqis and continuing problems with the economy make violent unrest more attractive - so every outbreak is worse than the last.
Sheila Armani of the BBC's Arabic service says: "The worst thing that could happen is for the rest of the Shias to feel that Mr Sadr has observed the ceasefire and has been attacked by the US.
"Then they will want to protect their co-religionist," she said.
"And then what's happening in Falluja and the Sunni triangle, could look like a very small fight compared to this."
Iraq's interim leader Iyad Allawi, himself a Shia Muslim, has tried to move towards an accommodation with Mr Sadr.
But his decision to lift a banning order on Mr Sadr's newspaper was scorned when Mr Sadr made his first appearance at Friday prayers in the mosque in Kufa at the end of July.
The Shia militia are said to be a small but well-trained force
Mr Sadr is the son of one of Iraq's most revered Shia leaders, who was almost certainly killed by Saddam Hussein. Both he and Ayatollah Sistani score highly in opinion polls.
With Ayatollah Sistani out of the way, even temporarily, Mr Sadr will expect to increase his support, both in the sprawling suburb of Baghdad which is named after his father, and in the southern cities where his Mehdi army have increasing power.
As well as the renewed fighting in Najaf, British forces are facing an upsurge of violence in Basra.