Only a minority of Shia were involved in recent violence
There has been Shia unrest in both Baghdad and the southern city of Basra.
More than 40 Shias and a number of coalition soldiers have been killed in the violence over the past 24 hours.
It was not supposed to be like this.
The Shia were, above all, the people the US and Britain came to Iraq to liberate.
They perhaps had claim to have suffered most under Saddam Hussein - killed in their thousands during the failed uprising of 1991, and stopped on pain of death from practising their most sacred rituals.
Now, for the first time in Iraq's history as a nation state, the Shia have the chance of winning the political power to match their majority in the population.
And yet, for a coalition which already has enemies enough in Iraq, a new front is opening up, with helicopter gunships over the Shia slums of Baghdad, and automatic fire in the holy city of Najaf, a place which had been calm for almost the whole of the past 12 months.
Ayatollah Sistani has been calling peaceful resistance
What has gone wrong? Well, the first and most important thing to stress is that this is not a general Shia uprising.
The trouble is coming from supporters of the radical young cleric, Moqtada Sadr, who has 10%, maybe 15% of Shias behind him.
There are also many secular Shias, and most religious Shia still cleave to the moderate Grand Ayatollah Ali al-Sistani.
He does not like the occupation either, but he is a quietist, who believes in peaceful opposition to the US.
The crucial task now facing the coalition is to stop the anger among this small group of radicals from turning into more general discontent.
That is a possibility, if, come 1 July and the handover of sovereignty to a new Iraqi government, the Shia feel somehow cheated of power.
It might also happen if the Shia believe that British and US forces are going to stay on indefinitely - that Iraqi self-rule is a sham.
I spoke to three masked members of a Shia militia in Basra last summer.
As they cradled rocket-propelled grenades in their arms, they said they were grateful to British soldiers for liberating their country, but if the British did not leave soon, they would start killing them.
There is, too, a third potential problem: that rivalry between Shia factions will become so vicious that coalition forces will get sucked in.
The moderate Shia cleric whom the US had pinned its hopes on in Iraq was murdered on the steps of the holy shrine in Najaf, in the first week after Saddam Hussein's forces were defeated.
Following the latest unrest, US generals are said to be thinking about requesting more troops.
Whatever the misgivings of the politicians about a greater commitment, the generals do not want the Shia violence to gain momentum.
In the long run, whether the British- and American-led coalition do win the war in Iraq, that will depend on the Shia.