Grim test for Iraq as Islamic State fighters dig in for battle of Fallujah

The scene of some of the bloodiest fighting in Iraq since Saddam Hussein's overthrow in 2003, Fallujah has been under IS control since January 2014. Just 65 kilometres west of the capital, its recapture might have been elevated on Baghdad's "must do" list some time back.

But all things are relative – and until there was a new, double-edged security threat in Baghdad, Iraqi Prime Minister Haider al-Abadi, like his predecessor Nouri al-Maliki and their American sponsors, had been happy to let Fallujah fester, despite its symbolic value as one of the last two major Iraqi cities in the territory that IS has dubbed its "caliphate".

With his government now rudderless and under pressure, Abadi needs to be seen to be in control. So he struts in the all-black fatigues of the country's elite counter-terror forces; he orders up a new military assault; and he goes on late-night television to promise that "the Iraqi flag will be raised high over the land of Fallujah".

Security threat No 1 in Baghdad is the recent advent of wild, mass protests on Fridays over the political gridlock, corruption and damned awful power and water services, in which followers of the Shiite cleric Muqtada al-Sadr have breached the security perimeter of Baghdad's high-security Green Zone – and last week, even managed to invade Abadi's office. Since the protests began, other Shiite movements have also taken to the streets, lest Sadr and his followers steal a march on them.

Security threat No 2 is a ferocious new wave of suicide attacks that have killed almost 200 people in predominantly Shiite areas of the capital. While analysts read these bombings as a bid by IS to draw Abadi's forces back to the capital and away from efforts to recapture parts of northern and western Iraq held by the jihadists, Baghdad residents hold Abadi's chaotic security services just as responsible as IS for the attacks.

There is also Security threat No 3 – the risk that another failure by Iraq's official security forces would reveal Abadi's dependence on the Shiite militias funded, trained and armed by neighbouring Iran – for which reason they are shunned by Washington.

Launched by Abadi on Sunday, the new war on Fallujah promises to be tough. The reputations of all the players in the Iraq crisis are on the line: Abadi and his administration; the Iraqi security forces and their American and Australian trainers; more than a dozen Shiite militia groups and their Iranian sponsors; and the IS forces that hold Fallujah.

The role of the Shiite militias is unclear. There are reports that they have been ordered to remain on the perimeter of the battle zone, where they have been heard chanting sectarian slogans, to allay fears based on their record of human rights abuses against Sunnis elsewhere in the country.

But sitting close to Abadi in images released by the Prime Minister's office after he visited the frontline was Hadi al-Amiri,  whose Badr Organisation - one of the oldest of the Shiite militias - controls Iraq's Interior Ministry.

Also seen in photographs, purportedly take in the "Fallujah operations room", is no less a figure than Qassem Soleimani, head of the Quds Force, the expeditionary arm of Iran's Revolutionary Guards. His presence at various fronts in the war on IS is a reliable signal both of intent to prevail and of a more serious role for the Shiite militias that he grooms.

In December, Iraqi forces managed to retake Ramadi, between Fallujah and the Jordanian border, but at significant cost – thousands of homes and other buildings were destroyed to end IS's year-long hold on the town. Ramadi is reportedly uninhabitable and more than 100 civilians died in attempts to return to the "liberated" city.

But the overlapping of the Fallujah operation with early planning for the long-delayed recapture of Mosul, Iraq's second biggest city in the north of the country, might be too big a challenge for the Iraqi security forces, which imploded as IS marched on Mosul in June 2014, much to the embarrassment of Baghdad and Washington.

The US military estimates that IS has lost about 45 per cent of its territory in Iraq in the last year, and that it has up to 700 fighters in Fallujah, who are expected to dig in for the long haul rather than cutting their losses and fleeing, as IS forces holding Ramadi and the city of Tikrit did. Abadi's force reportedly numbers up to 50,000.

Fallujah, which has a peacetime population of more than 250,000, has been under siege for months. The United Nations estimates that only 60,000 to 100,000 people remain in the Sunni stronghold long associated with resistance to the armies of Washington and then Baghdad – thereby making them suspect to Iraq's majority Shiite population.

However, the city is under IS lockdown - the group has reportedly posted snipers on main routes in and out of the city, all of which have been made impassable with improvised explosive devices. Supplies of food and water are said to be low.

The UN estimates that only 80 families have been able to escape and as many as 21 have reportedly died as Abadi's combined force of troops, police, militias and Sunni tribal fighters have mustered on the outskirts and begun to bombard the city without any apparent attempt to enter it.

Some years ago, Australian counter-terrorism expert David Kilcullen explained to me that it was not the level of terrorist violence that made a country ungovernable, so much as the capacity of the government to deal with the violence.

By that measure the new battle for Fallujah, if Abadi and his allies persists, might prove instructive as a measure of who's in charge in Iraq.

But things are so volatile in Baghdad these days that Abadi has pleaded for the Friday protesters to stay at home this weekend – because the security forces can't walk and chew gum at the same time.