Violence has exploded in Iraq after some months of relative quiet.

For the past three days, Iraqi troops have targeted Shiite militias in the southern port city of Basra, with militiamen reportedly using mortar shells, sniper fire, roadside bombs and rocket-propelled grenades to repel security forces. Since Wednesday, at least 56 people have been killed.

Responding to the government crackdown on Shiite militias in Basra, Muqtada al-Sadr’s Mahdi Army is pounding Baghdad’s U.S.-protected Green Zone with rocket fire. The security operation has triggered a wave of Shiite violence through southern Iraq and in Baghdad, and thousands of protestors have demanded Iraqi Prime Minister Nouri al-Maliki resign. U.S. Embassy staff in Baghdad have been advised to wear helmets and other protective gear if they venture outside, and to sleep in blast-resistant locations. The Iraqi spokesman for the Baghdad security operation has been kidnapped and three of his bodyguards killed.

Unsurprisingly, Pentagon officials report that the weapons being used to attack the Green Zone include rockets made in Iran.

The security operation is an attempt by Iraq’s central government to gain control of the critical oil hub of the nation. Basra, Iraq’s second-largest city, is critical to the country’s oil exports, its only source of income: 90 percent of Iraqi crude is exported through the city’s infrastructure. Highlighting the vulnerability of Iraq’s oil-dependent economy was the bombing on Thursday of a major crude export pipeline in Basra, causing the main pumping station to be shut down and reducing oil production and movement by one third until the pipeline is repaired.

Basra has been under control of rival Shiite militias pretty much since the U.S. invasion of Iraq. British troops, who withdrew from Basra late last year, were never able to wrest control of the city from the militias. Now, with security in the region deteriorating, the Iraqi government has acted to gain control of the region.

In a speech broadcast on Iraqi state tv, Prime Minister al-Maliki said Iraq had become a “nation of gangs, militias and outlaws” and he was undertaking a “historic mission” in Basra to restore “the law of the land.”

Al-Maliki has ordered Mahdi Army militiamen to surrender by the weekend; as of Thursday, they were still forcefully resisting the crackdown and still controlling the streets of Basra.

Stratfor reports that the operation is in the interests both of the country’s most powerful Shiite movement, the Islamic Supreme Council of Iraq (isci) led by Abdel Aziz al-Hakim, and Iran (March 26):

Targeting the chief rivals of the isci, which seeks to establish an autonomous Shiite region in the south, gives al-Hakim’s group an opportunity to consolidate itself in what is perhaps the most important part of the Shiite south. …Stability in Iraq’s Shiite areas also better serves Tehran’s long-term interests; a southern federal zone could help achieve such stability.

The results of this current security operation could have great impact on the future of U.S. forces in Iraq. It could help prove whether the U.S. troop surge in Iraq has, in the longer term, been a success or failure. Violence has indeed greatly decreased over recent months in Iraq; but this is largely as a result of the unilateral cease-fire of al-Sadr’s Mahdi Army seven months ago. The test is now—when the militia is starting to fight back.

If the Iraqi government demonstrates it has the ability to, at least in part, bring a festering insurgency under control, America will have the rationale it is looking for to hasten troop withdrawal from the country. On the other hand, if the Shiite militias win out, the United States will be all the more forced to rely on Shiite Iran to reign in Iraq’s Shiites. Either way, it is a situation worth watching.

We can expect Iran to try to use this as a means of furthering its influence in Iraq. Read our March 13 article “Troop Surge in Iraq: Success or Failure?” for a longer-term perspective on the Iraqi situation’s impact on America.