Tens of thousands of Shia militiamen are poised to join the battle for Fallujah, 40 miles west of Baghdad, in a bid to recapture it from Isis fighters who seized the city 18 months ago. The battle is likely to be one of the decisive military engagements of the Iraq war as Fallujah has been at the centre of the Sunni revolt in Iraq since the US invaded the country in 2003.

“Fallujah is surrounded, but we will take it little by little,” Brigadier-General Ali Musleh, a senior commander of the Shia militia force known as the Hashid Shaabi, told The Independent as Iraqi armed forces pressed Fallujah on three sides. Iraqi forces say they are keeping open a corridor through which civilians can escape before ground fighting escalates.

In a brief statement, Iraq’s Prime Minister, Haider al-Abadi, vowed to “take revenge from Daesh [Isis] criminals on the battlefield … and their cowardly crimes against unarmed civilians will only increase our determination to chase them and to expel them from the land of Iraq”.

The Hashid Shaabi is made up of volunteers who answered a call to arms by Grand Ayatollah Ali al-Sistani after Isis captured Mosul on 10 June 2014. The Iraqi government announced the beginning of the operation to retake the giant Sunni province of Anbar that is 85 per cent held by Isis. Hashid forces match Isis fighters in their fanatical willingness to fight to the death. They are numerous, inspired by religious faith and have been successful in fighting around Baghdad. But their failings include a lack of experienced commanders and training, leading to heavy casualties.

Timeline: The emergence of Isis Show all 40 1 /40 Timeline: The emergence of Isis Timeline: The emergence of Isis 2000 Abu Musab al-Zarqawi (pictured here) forms an al-Qaeda splinter group in Iraq, al-Qa’eda in Iraq. Its brutality from the beginning alienates Iraqis and many al-Qaeda leaders. Timeline: The emergence of Isis 2006 Al-Zarqawi is killed in a U.S. strike. Al-Zarqawi’s successor, Abu Ayyub al-Masri, announces the creation of the Islamic State in Iraq (ISI). Reuters Timeline: The emergence of Isis 2009 Still al-Qaeda-linked ISI claims responsibility for suicide bombings that killed 155 in Baghdad, as well as attacks in August and October killing 240, as President Obama announces troop withdrawal from Iraq in March. Getty Images Timeline: The emergence of Isis 2010 Abu Bakr al-Baghdadi becomes head of ISI, at lowest ebb of Islamist militancy in Iraq, which sees last U.S. combat brigade depart. Timeline: The emergence of Isis 2012 In Syria, protests (pictured here starting in Daree) have morphed into what president Assad labelled a “real war” with emergence of a coalition of forces opposed to Assad’s regime. Syria group Jabhat al-Nusra are among rebel groups who refuse to join, denouncing it as a “conspiracy”. Bombings targeting Shia areas, killing more than 500 people, spark fears of new sectarian conflict. Sunni Muslims stage protests across country against what they see as increasingly marginalisation by Shia-led government. AP Timeline: The emergence of Isis 2013 Al-Baghdadi renames ISI as the Islamic State in Iraq and Syria, or Isis, as the group absorbs Syrian al-Nusra, gaining a foothold in Syria. In response, al-Qaeda chief Ayman al-Zawahiri (Bin Laden’s successor) concerned about Isis’ expansion orders that Isis be dissolved and ISI operations should be confined to Iraq. This order is rejected by al-Baghdadi. AFP Timeline: The emergence of Isis 2014 - January Isis fighters capture the Iraqi cities of Fallujah and Ramadi, giving them base to launch slew of attacks further south. AP Timeline: The emergence of Isis 2014 - June Isis declares itself the Caliphate, calling itself Islamic State (IS). The group captures Mosul, Iraq’s second largest city; Tal Afar, just 93 miles from Syrian border; and the central Iraqi city of Tikrit. These advances sent shockwaves around the world. Timeline: The emergence of Isis 2014 - June Around the same time Isis releases a video calling for western Muslims to join the Caliphate and fight, prompting new evaluations of extremists groups social media understanding. Timeline: The emergence of Isis 2014 - June Isis take Baiji oil fields in Iraq - giving them access to huge amounts of possible revenue. EPA Timeline: The emergence of Isis 2014 - August James Foley is executed by the group as concerns grow for second American prisoner, fellow reporter Steven Sotloff. AP Timeline: The emergence of Isis 2014 - August Obama authorises U.S. airstrikes in Iraq, helping to stall Isis’ along with action by Kurdish forces following the deaths of hundreds of Yazidi people on Mount Sinjar. Timeline: The emergence of Isis 2014 - September Isis release video showing Steven Sotloff’s murder prompting Western speculation his executioner is same man who killed Mr Foley. EPA Timeline: The emergence of Isis 2014 - September Obama tells us that America “will hunt down terrorists who threaten our country” EPA Timeline: The emergence of Isis 2014 - September Isis release a video appearing to show David Haines, who was captured by militants in Syria in 2013, wearing an orange jumpsuit and kneeling in the desert while he reads a pre-prepared script. It later shows what appears to be the aid worker's body. Rex Timeline: The emergence of Isis 2014 - September Peshmerga fighters scrabble to hold positions in the Diyala province (a gateway to Baghdad) as Isis fighters continue to advance on Iraqi capital. AFP Timeline: The emergence of Isis 2014 - October Aid worker Alan Henning is killed. Self-imposed media blackout refuses to show images of him in final moments, instead focuses upon humanitarian care. AP Timeline: The emergence of Isis 2014 - October Isis raise their flag in Kobani, which had been strongly defended by Kurdish troops. The victory goes against hopeful western analysis Isis had overextended itself, while alienating much of the Muslim population through the murder of Henning. Victory causes fresh waves of Kurdish refugees arriving in Turkey. Timeline: The emergence of Isis 2014 - November American hostage, who embarced values of Islam, Peter Kassig and 14 Syrian soldiers are shown meeting the same fate as other captives. But intelligence agencies will be poring over the apparently significant discrepancies between this and previous films. Seramedig.org.uk Timeline: The emergence of Isis 2015 - February Isis has released a video revealing the murder by burning to death of a Jordanian pilot held by the group since the end of December 2014. Reuters Timeline: The emergence of Isis 2015 - February Isis militants have released videos which appear to show the beheading of Japanese hostages Haruna Yukawa and Kenji Goto. Timeline: The emergence of Isis 2015 - February American aid worker, Kayla Mueller was the last American hostage known to be held by Isis. She died, according to her captors, in an airstrike by the Jordanian air force on the city of Raqqa in Syria, though US authorities disputed this. AP Timeline: The emergence of Isis 2015 - February Isis militants have posted a gruesome video online in which they force 21 Egyptian Coptic Christian hostages to kneel on a beach in Libya before beheading them. Egypt vowed to avenge the beheading and launched air strikes on Isis positions. AP Timeline: The emergence of Isis 2015 - February The British Isis militant suspected of appearing in videos showing the beheading of Western hostages has been named in reports as Mohammed Emwazi from London. Rex Features Timeline: The emergence of Isis 2015 - March Isis triple suicide attack has killed more than 100 worshippers and hundreds of others were injured after the group members targeted two mosques in the Yemeni capital of Sanaa. AP Timeline: The emergence of Isis 2015 - April Iraqi forces have claimed victory over Isis in battle for Tikrit and raised the flag in the city. EPA/STR Timeline: The emergence of Isis 2015 - April Isis has claimed responsibility for a suicide bomb attack in Afghanistan that killed at least 35 people queuing to collect their wages and injured 100 more. EPA Timeline: The emergence of Isis 2015 - April Isis’ media arm released a 29-minute video purporting to show militants executing Ethiopian Christians captives. The footage bore the extremist group’s al-Furqan media logo and showed the destruction of churches and desecration of religious symbols. A masked fighter made a statement threatening Christians who did not convert to Islam or pay a special tax. Timeline: The emergence of Isis 2015 - May Abu Bakr al-Baghdadi, the leader of Isis has been "incapacitated" by a spinal injuries sustained in a US air strike in Iraq. He is being treated in a hideout by two doctors from Isis’ stronghold of Mosul who are said to be "strong ideological supporters of the group". Timeline: The emergence of Isis 2015 - May Isis has also claimed responsibility for killing 300 of Yazidi captives, including women, children and elderly people in Iraq AP Timeline: The emergence of Isis 2015 - May Isis attack on Prophet Mohamed cartoon contest in Texas was its first action on US soil. Two gunmen were shot and killed after launching the attack at the exhibition. Elton Simpson and Nadir Soofi have been named as the attackers at the Curtis Culwell Centre arena in Garland. Timeline: The emergence of Isis 2015 - May Isis’s deputy leader, Abu Alaa Afri, a former physics teacher who was thought to have taken charge of the deadly terrorist group, has been killed in a US-led coalition airstrike. Timeline: The emergence of Isis 2015 - May US special forces have killed a senior Isis leader named as Abu Sayyaf in an operation aiming to capture him and his wife in Syria. Getty Images Timeline: The emergence of Isis 2015 - May Iran-backed militias are sent to Ramadi by the Iraqi government to fight Isis militants who completed their capture of the city. Government soldiers and civilians were reportedly massacred by extremists as they took control and the army fled. Charred bodies were left littering the city streets as troops clung on to trucks speeding away from the city. Ramadi is the latest government stronghold to fall to the so-called Islamic State, despite air strikes by a US-led international coalition aiming to stop its advance in Iraq and Syria. AFP Timeline: The emergence of Isis 2015 - May Isis rounded up civilians trapped in Palmyra and forced them to watch 20 people being executed in the historic city’s ancient amphitheatre. The Unesco World Heritage site was overrun by militants, threatening the future of 2,000 year-old monuments and ruins. Thousands of Palmyra’s residents fled but many are still living within the city walls, while the UN human rights office in Geneva said it had received reports of Syrian government forces preventing people from leaving until they retreated from the city. Getty Timeline: The emergence of Isis 2015 - May A group of Isis-affiliated fighters have captured a key airport in central Libya. The militants took control of the al-Qardabiya airbase in Sirte after a local militia tasked with defending the facility withdrew from their positions. Affiliates of Isis, already control large parts of Sirte, the birthplace of former Libyan leader Muammar Gaddafi and a former stronghold of his supporters. Timeline: The emergence of Isis 2015 - June The US Air Force has destroyed an Isis stronghold after an extremist let slip their location on social media. According the Air Force Times, General Herbert "Hawk" Carlisle, commander of Air Combat Command, said that Airmen at Hulburt Field, Florida, used images shared by jihadists to track the location of their headquarters before destroying it in an airstrike. Reuters Timeline: The emergence of Isis 2015 - June Kurdish forces captured a key military base in a significant victory in Raqqa as well as town of Tell Abyad. YPG fighters, backed by US-led airstrikes and other rebels, consolidated their gains, when they seized the key town on the Syria-Turkey border. They are now just 30 miles to the north of Raqqa and have cut off a major supply route deep inside Isis-held territory. Ahmet Silk/Getty Timeline: The emergence of Isis 2015 - June Isis has released gruesome footage claiming to show the murder of more than a dozen men by drowning, decapitation and using a rocket-propelled grenade as it seeks to boost morale among its fanatical supporters. Timeline: The emergence of Isis 2015 - June Isis has begun carrying out its threat to destroy structures in the ancient Syrian city of Palmyra, blowing up at least two monuments at the Unesco-protected site as Syrian government troops made advances on the Islamist’s positions. AFP

The Iraqi army was weakened and discredited when it unexpectedly lost Ramadi, the capital of Anbar province, to Isis fighters on 17 May this year. The Shia-dominated government badly needs a victory against Isis and has no alternative but to use Shia militia units.

In the build-up to the battle Hashid military leaders have been leaving the holy city of Karbala, site of two of the greatest Shia shrines, for the front line around Fallujah, 54 miles to the north.

“The fighting has already started and I have been told to be at the front by tomorrow morning,” a Hashid member said late last night.

Isis fighters celebrating in Fallujah, which the militants took in 2014 (AP)

Fallujah was twice besieged by US Marines in 2004, their forces finally storming the city after prolonged bombardment by air strikes and heavy artillery. Isis has had a long time to plant IEDs and booby traps, which it has used to great effect in the past. Its tactics include well-trained snipers, mortar teams and suicide bombers, the latter deployed in large numbers and driving vehicles that have been specially armoured and packed with explosives.

The poor training of Hashid forces is confirmed by Colonel Salah Rajab, who was deputy commander of the Habib Battalion of the Ali Akbar Brigade, a 300-strong unit whose soldiers come from Karbala. He was so badly wounded by the blast from a mortar bomb on 3 July that his lower right leg has been amputated. Looking cheerful and resolute despite his injury, he explains that he is a 55-year-old military professional who was a colonel in the old Iraqi army from which he resigned in 1999. He says: “I joined the Hashid to defend my country against foreigners and I had been fighting in Baiji city for 16 days when a mortar bomb landed near me, leaving two of our men dead and four wounded.”

Lying in bed in the Hussein Teaching Hospital in Karbala in a ward full of wounded Hashid fighters, Col Rajab lists the main weaknesses of his men. He says that “there is a lack of experienced commanders who know how to direct an attack, though they do ask advice from professional military officers and usually listen to us”.

There is a general lack of training for the volunteers, making them less efficient and liable to suffer losses, Col Rajab says, adding that “they get a maximum of three months’ training when they need six months”. This lack of experience and training not only affects tactics, but security. He says that “Daesh [Isis] is continually hacking into our communications, finding out what we are doing and inflicting heavy losses”.

These points are borne out by the stories of other wounded Hashid fighters in the hospital. Omar Abdullah, a thin-looking 18-year-old with a heavily bandaged broken arm and leg, says he had just 25 days’ training before being committed to battle in Baiji. What happened next shows the sophistication of Isis’s battlefield skills.

Mr Abdullah says: “We were shot at by snipers and we ran into a house to seek cover. There were 13 of us and we didn’t realise that the house was full of explosives.” No sooner had they fallen into the trap than Isis detonated the bombs in the house, killing nine and wounding four of the fighters. What did Mr Abdullah think of the short training period? He says: “It should be more, but we need to fight as soon as possible.”

Isis has always relied heavily on IEDs, booby traps and suicide bombers to slow the advance of their enemies and cause casualties. Fadil Rashid is a bomb disposal expert from Nassiriya, south-east of Baghdad, who was on patrol north of Samara in Salahuddin province when he saw a suspicious-looking bridge about 15ft long over a canal. Before he could prevent him, one of his men put a foot on the bridge, detonating a bomb that killed four and wounded three men.

It seems likely that Isis will fight for Fallujah in a way that it did not fight for Tikrit, Saddam Hussein’s home town, which fell on 1 April after a month’s siege to an assault led by Shia militiamen. But Isis did not fight hard to hold it while preparing for a surprise counter-attack at Baiji and Ramadi.

Fallujah is more important because it was the first Iraqi city to be taken over by the militants, in January 2014. It is close to Baghdad and poses a threat to the capital. An early sign of the military weakness of the Iraqi government early last year was its inability to launch a successful counter-offensive to recapture Fallujah.

If Fallujah does fall then it will be a mixed blessing for the Iraqi government, which has only limited control over the Shia militias that are now its main fighting force. The regular army has only about between 10,000 and 12,000 reliable combat troops. These are, however, reported to be exhausted and fought-out by being rushed from crisis to crisis without rest.

An important feature in the battle for Fallujah will be the willingness of the US to use its air power to support the Hashid attack. At Tikrit and Ramadi the US insisted that its air power be used only to support regular Iraqi army troops. The US sees several of the Shia militias as being under the control of Iran, though there are others more closely allied to the government.

Yesterday, a delivery of 36 F-16 fighter jets purchased by the Iraqi government was reported to have arrived at Balad air base, north of Baghdad.