Insurgents from the Islamic State in Iraq and Syria (ISIS) have this week captured one province, including two cities and numerous towns in Iraq, bringing the radical Sunni Islamist group sharply into focus.
However, the one-time affiliate of al-Qaeda has been operating since shortly after the ill-fated 2003 US invasion of Iraq in one guise or another.
ISIS is also known as the Islamic State in Iraq and Levant (ISIL) or Islamic State of Iraq and Al-Sham, and the group's aim is to create an Islamic caliphate encompassing Iraq, Syria and Lebanon.
It aims to do this primarily by fomenting sectarian conflict with Shia Muslims. In Iraq, Sunnis make up around 25% of the population, while in neighbouring Syria, where the group is fighting Bashar al-Assad’s regime, Sunnis make up about 70%.
Founded in the wake of the US-led invasion of Iraq by Jordanian militant Abu Musab al-Zarqawi in 2004, the group declared allegiance to al-Qaeda and its then leader Osama bin Laden and was known as al-Qaeda in Iraq.
Al-Zarqawi, who would go on to declare all-out war against the Shia in Iraq, was ultimately killed by a US strike in June 2006.
The hallmarks of al-Qaeda in Iraq were laid down before his death, however. These include car bombings and suicide bombings in crowded markets, mass executions and targeting of Shia neighbourhoods.
One of the most influential acts by the group was the 2006 al-Askari Mosque bombing, which destroyed its golden dome and severely damaged the mosque.
One of the holiest sites in Shia Islam, the bombing served to be the precursor to two of the bloodiest years since the invasion, with thousands of Iraqis killed and the country effectively being in the throes of a civil war.
The group’s influence declined however with the creation of the US-sponsored ‘Sunni awakening’, coalitions between tribal Sheikhs that united to maintain security in their communities.
Al-Qaeda in Iraq condemned the "awakening" movement for siding with the US and Shia-led Iraqi government.
However, by 2011 both US strikes against its leaders and the fact that up to 100,000 Sunnis joined the awakening councils meant that the group had become largely irrelevant.
Since 2011 however, the group changed tactics. Instead of simply fighting as an al-Qaeda offshoot it started to operate as a proto-state.
It imposed taxes on the territory it controlled, and imposed its own form of justice based on its strict interpretation of Islam – for example no smoking, music, sports or unveiled women.
However, it was the Arab Spring and resulting civil war in Syria that has boosted ISIS both in terms of territory and manpower.
Syrian President Bashar al-Assad has largely left ISIS alone, primarily because of his assumption that ISIS would squeeze out the more moderate rebels in the country such as the Free Syrian Army and also because it would give his regime more legitimacy in the eyes of the Western countries that oppose him.
This, coupled with the fact that Shia Nouri al-Maliki disbanded the Sunni awakenings movement, refused to integrate them into the Iraqi army and purged leading Sunnis from government, gave the group a fresh impetus.
2013 proved to be a breakout year for the militant group. It changed its name to the Islamic State in Iraq and the Syria, or al-Sham, or the Levant depending on your interpretation.
The group organised a mass breakout of 500 detainees in Iraq’s notorious Abu Graib prison. This was part of a year-long ‘Breaking the Walls’ campaign aimed at freeing its members from prisons.
As well as boosting militants from jails, the group has been further bolstered by an influx of foreign fighters to the conflict in Syria.
The danger for Western countries of these fighters was highlighted recently when an attack on a Jewish museum in Brussels left four people dead.
The suspected gunman was reportedly in possession of a Kalashnikov rifle, a handgun – and an ISIS flag.
Disowned by al-Qaeda
In late 2013 the new al-Qaeda leader Ayman al-Zawahiri attempted to disband the group – withdrawing his support in favour of the Al-Nusra Front.
However, ISIS leader Abu Bakr al-Baghdadi dismissed the action and ISIS has since gone on to carry out sporadic killings in clashes with Al-Nusra.
In January of this year the group announced it had captured the Sunni city of Fallujah – which has witnessed some of the heaviest fighting in Iraq since 2003.
Also in January it carried out car bombings in the Lebanese capital Beirut and announced the formation of a Lebanon chapter – promising to fight the Shia Muslim Hezbollah and its supporters.
In recent months anti-ISIS rebels have made solid gains against the group in Syria, while conversely it is in Iraq where the group has made its biggest advances.
In recent days it has captured both Iraq’s second largest city, Mosul, and Tikrit, the home town of Iraq’s most notorious son, Saddam Hussein.
This video, uploaded to YouTube on Tuesday, appears to show the city of Mosul under the control of the militants.
Abandoned vehicles of government forces are in flames on the streets and fighters are seen roaming the city in pickup trucks.
The group has been much bolstered by the Iraqi army’s feeble response to the insurgents and al-Maliki’s apparent indifference to the group until this week’s events.
The group's lightning assault continues, with its forces advancing towards Baghdad.
It has been reported the group has now become the richest insurgent groups ever after looting 500 billion Iraqi dinars ($429m) from Mosul's central bank.
A large quantity of gold bullion is also thought to have been stolen.
Even more worryingly, top Shia cleric Grand Ayatollah Ali al-Sistani has called on Iraqis to take up arms against the insurgents and Shia Iran has indicated it will also not stand idly by.
More than a decade after its inception, Abu Musab al-Zarqawi’s goal of creating a civil war in Iraq between Sunnis and Shias looms ever closer.