The Islamic State of Iraq and al Shams (ISIS) is a militant terrorist group and off-shoot of al-Qaeda.
ISIS has its origins in the insurgency against US forces in Iraq and was originally led by Abu Musaib al-Zarqawi. Known then as the Islamic State of Iraq, the group was brought to the brink of collapse by the killing of al-Zarqawi in a US airstrike in June 2006 and the intensive US counterterrorism campaign that seen US forces align with Sunni tribes.
Abu Bakr al-Baghdadi is the current leader of the group, and used the conflict in Syria to revitalise the movement.
Al-Baghdadi's plan is to create an Islamic emirate based on the rule of Sharia law across the region.
In Syria, ISIS has enjoyed a quick rise to prominence, dominating large parts of northern Syria from which it strengthens its position and attempts to attract new recruits.
ISIS has made some attempts to win the hearts of the local populations by providing supplies, however its enforcement of a brutal version of Sharia law has left it deeply unpopular with many Syria civilians. It is famous for ordering executions, beheadings and amputations in the areas it controls in eastern Aleppo and the city of Raqqa.
The extremity of their methods prompted al-Qaeda's central command to disown them, saying they were unnecessarily violent towards their fellow Muslims.
ISIS have used their territories in Syria to re-launch into Iraq. Over the past year the group has slowly been working to take advantage of the dissatisfaction in Sunni areas with the Shia dominated government and military.
In January this year, ISIS successfully took over Fallujah, the Iraqi city to the west of Baghdad.
Having been openly active in Mosul since last summer, on June 10th ISIS successfully captured large stores of Iraqi army weaponry and hardware.
ISIS is now operating in Samarra and Tikrit to the north of Baghdad, and Ramadi to the west of the city.
As the fighters paraded through Iraq's second city of Mosul yesterday, the vowed to march towards Baghdad.
If they launched a sustained offensive on the capital and managed to take the city whilst maintaining control of the territories already captured, ISIS would control a territory from Aleppo to Baghdad.