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The rapid advance across Syria and Iraq by militant fighters from Islamic State (IS) in 2014 threw the region into chaos and led to US air strikes against their key positions.
By June, the jihadist group, which has fighters from across the world, announced the establishment of a "caliphate" - an Islamic state - stretching from Aleppo in Syria to the province of Diyala in Iraq.
The US went on to assemble a coalition to fight the militants, and has so far launched more than 1,300 air strikes against IS targets in Iraq since the campaign began on 8 August. Meanwhile, the UK launched its first air strikes on 30 September.
In neighbouring Syria, the US, along with Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates, has also carried out more than more than 1,000 attacks on IS-held areas since 23 September 2014.
US President Barack Obama has warned his coalition allies they are facing a "long-term campaign".Militants from abroad
The US Central Intelligence Agency believes IS may have up to 31,000 fighters in the region, many of whom are foreign recruits.
Figures from the London-based International Centre for the Study of Radicalisation and Political Violence (ICSR) and the New York-based Soufan Group show an estimated 20,000 fighters from almost 80 countries have travelled to Syria and Iraq to fight with extremist groups.
The figures suggest that while about a quarter of the foreign fighters are from the West, the majority are from nearby Arab countries, such as Tunisia, Saudi Arabia and Jordan and Morocco.Refugees
Almost four million people have fled abroad to escape the fighting in Syria. Most have gone to Lebanon and Turkey - but a significant number have also gone to Iraq.
Syrian refugees have put pressure on local services and infrastructure in Iraq - which is also having to cope with the return of many Iraqi refugees from Syria.
In addition, the UN estimates there are more than 2m Iraqis who have been forced to leave their homes to escape the conflict and are displaced within the country or elsewhere.Battlegrounds
The latest key battleground is around the Iraqi city of Tikrit, the home town of former president Saddam Hussein. Pro-Iraq government forces are reported to be closing in on the city centre, which has been held by IS for more than eight months.
The Iraqi army is being supported by Shia militia groups, who call themselves Popular Mobilisation.
Since 2014, a number of Shia militia groups have emerged in Iraq that openly pledge allegiance to Iran. General Qasem Soleimani, the commander of Iran's Islamic Revolution Guards Corps (IRGC) Quds Force, is taking part in leading the Tikrit operation.
If the battle for Tikrit is successful, the Iraqi army is expected to push northwards to Mosul.
Another battleground has been the fight for Kobane, a town close to the Syrian border with Turkey.
Thousands of residents were forced into Turkey to flee the fighting, while coalition airstrikes targeted the advancing Islamic State fighters.
After months of fighting, in which about 1,600 people died, US Central Command announced in January 2015 that anti-IS forces controlled 90% of the town.
Strategically for IS, Kobane's capture would have allowed the militants control of key border territory. But although the liberation of the town is a setback for the militant movement, it still controls large parts of northern and eastern Syria, as well as northern and western Iraq.
IS fighters targeted a number of Iraqi dams during their advance, capturing the facility at Falluja in April 2014. They went on to take Mosul dam in August, before US air strikes helped force them out later that month.
IS fighters also attacked the country's second largest dam at Haditha, but the area was secured by Iraqi forces In September.
In the course of its offensives in Iraq and Syria in June 2014, IS gained control of much of the oil infrastructure.
These refineries and the fields supplying them with oil have played a vital role in fuelling IS military units and in generating revenue for the group.
The coalition has targeted these locations in an attempt to damage IS capabilities.