As the government dithers on the thorny issue of allowing U.S. troops to stay on after the Dec. 31 withdrawal deadline, Tehran-backed Shiite "special Groups" and Sunni militants are squaring off for a showdown that could plunge Iraq into anarchy.

The Iranians' "Special Groups" -- Kataeb Hezbollah, the Promised Day Brigades and Asaib Ahl al-Haq -- were created in 2007 to wage a proxy war on Tehran's behalf against the Americans and their Iraqi allies.

They were part of a long line of Iranian-sponsored paramilitary proxies wielded by the Tehran regime from the moment it took power in 1979.

Tehran reined in the Iraqi units after U.S. forces hit them hard but the groups, armed, trained and funded by the Revolutionary Guards' elite Al-Quds Force, its covert action arm, remained intact with "plausible deniability" as instruments of state policy.

They were recently reactivated to serve Iran's strategic agenda as U.S. forces withdrew, including waging war on the Baghdad government if it seeks to become too independent of Tehran and maintain relations with the United States.

The recent deal by the government of Prime Minister Nouri al-Maliki, to buy 18 Lockheed Martin F-16 fighter aircraft, possibly the first of up to 96, indicates he wants to maintain military links with Washington for some time to come.

"Since 2003, Iran has pursued three political objectives in Iraq," the Washington Institute for Near East Policy remarked in a May analysis.

"First, it has sought to unite the country's Shiite parties so they can translate their demographic weight -- an estimated 60 percent of Iraq's population -- into political influence, thereby creating a government amenable to Tehran …

"Second, Iran has supported shaky governing coalitions, positions itself to be an influential external power broker when the need for mediation arises.

"Third, Tehran has sought to prevent non-Islamist parties from gaining power … To that end, it has also worked to marginalize secular nationalist factions and leaders," the institute noted.

On the Sunni side, a resurgent, third-generation al-Qaida in Iraq and the Baathist Jaish Rijal al-Tariq al-Naqshabandi, or JRTN, are leading a revival of Sunni militancy that feeds on Sunni fears of Shiite and Iranian repression.

A retooled AQI has surfaced in former jihadist strongholds in Baghdad and western Iraq following the loss of several key leaders to U.S. and Iraqi forces.

Among them was Atiyah Abdel Rahman, a Libyan who was a key link between AQI and al-Qaida Central in Pakistan, where he was killed Aug. 2 in a U.S. missile strike.

AQI is waging a campaign of violence across Iraq, attacking rival Shiite communities and security chiefs in mass-casualty suicide attacks that have killed hundreds of people.

After Osama bin Laden was assassinated by U.S. Navy SEALs in Pakistan May 2, AQI leader Ibrahim Awwad Ali al-Badri, aka Abu Du'a, announced the "Plan of the Good Harvest," a jihadist firestorm of 100 attacks across Iraq to avenge bin Laden.

The U.S. government tagged Badri "a specially designated global terrorist" Oct. 4, and slapped a $10 million bounty on his head, making him one of the three most-wanted terrorist fugitives in the world.

JRTN -- whose Arabic name means "Army of the Men of the Naqshabandiya Order" -- is led by Saddam Hussein's former henchman Izzat Ibrahim al-Douri.

His impeccable Baathist credentials include being the last of the plotters who put the Sunni-dominated Baath Party in power in a 1968 coup.

The Naqshabandiya is a Sufi sect associated with the dervish culture. But, says Michael Knights of the Washington Institute, "the group is a chameleon that has at time stressed its Islamist credentials, tribal networks and old Arab socialist roots as circumstances required.

"Like its leader, al-Douri, and the Baath Party of old, JRTN can become whatever it needs to be to achieve its goals."

Knights, who specializes in Middle East security issues, says JRTN is playing a key role in a new Sunni insurgency that's taking shape.

It includes Saddam-era military intelligence officers and "is rapidly becoming the most influential insurgent group in the country."

It has gained strength as the once-powerful Sunni minority found itself adrift after backing former Prime Minister Iyad Allawi, a secular Shiite, in the March 2010 parliamentary election that eventually put al-Maliki, a Shiite of the Iranian-backed Ad-Dawa Party, back in power.