By Paul Wood
BBC Middle East correspondent
An Iraqi businessman recently sent a container of goods through Basra port. The cost was $500 in transport - and $3,000 in bribes.
The Mehdi Army is one of Basra's most powerful militias
The story is related by Patrick Cockburn, biographer of the radical Shia cleric, Moqtada Sadr, whose Mehdi Army militia is the main target of the big security operation in Basra.
Cockburn says, quite correctly, that a lot of the coalition's success in reducing violence in Iraq in recent months has to do with a ceasefire by the Mehdi Army.
There are now worrying signs that action against the militia in Basra and arrest raids in Baghdad are jeopardising the ceasefire.
The Iraqi government felt it had to act in Basra because much of the country's oil exports flow out through there as well as being the route in for many of Iraq's imported goods.
Choked by corruption
That economic lifeline is being choked by corruption and by the violence which accompanies it as rival criminal and political militias fight over the spoils.
Moqtada Sadr believes his followers will deliver him power
One militia, which has links to the Basra governor, controls the port. Other factions, but chiefly the Mehdi Army, regularly skirmish with them.
Everyday life, too, is dominated by the rule of the gun in Basra.
Some 100 women have been murdered by religious extremists over the past year for wearing make-up or Western style dress.
Local people associate most attacks like these with members of the Mehdi Army. The police are little help as they are heavily infiltrated by the militants.
The fact that the police are so compromised in Basra is one reason why thousands of Iraqi army troops have been sent down from Baghdad to take part in the operation.
Tuesday's fighting in Basra can be seen as the government trying to impose law and order - but also as part of the power struggle within the Shia community.
Moqtada Sadr believes his hundreds of thousands of followers, many of them armed, will eventually deliver power into his hands.
The Mehdi Army has vowed to step up attacks on "occupation forces"
Iraqi Prime Minister Nouri Maliki and his allies are determined to stop him.
In Basra, the British troops are staying out of this fight, saying the Iraqi army is demonstrating it is capable of acting on its own.
Further north in Baghdad, the Americans tend to act in support of the local security forces.
In reply, the Mehdi Army has promised to step up attacks on the "occupation forces".
The Americans are congratulating themselves at the moment on the success of the surge in averting a Sunni-Shia civil war - and over the thousands of former Sunni insurgents who have changed sides to help the coalition fight al-Qaeda.
But the lesson of Tuesday's events is that intra-Shia violence could be just as dangerous to hopes of peace as sectarian hatreds or the insurgency.