- ISLAMIC STATE
IS scored its biggest victory since the summer of 2014 when it took control of Ramadi on Sunday.
The jihadist group and its previous incarnations have maintained a presence in the Ramadi region for the past decade, even after US-led efforts to remove insurgents from Anbar. It has excellent knowledge of the terrain.
The jihadists, who for a year and a half had been unable to size Ramadi fully as they did with Fallujah further east, received reinforcements in recent weeks from fighters who lost Tikrit to the government.
IS fighters are well-equipped, thanks to equipment seized from the army in Syria, which Anbar borders, and from retreating Iraqi troops. Pictures released on Monday showed that the latest spoils include tanks.
The group has become expert at laying sophisticated booby traps that make any advance difficult and its use of suicide cars packed with explosives can cause huge casualties.
- GOVERNMENT FORCES
Facing IS are Iraqi security forces, including army troops stationed in bases a few kilometres (miles) from Ramadi, police forces and counter-terrorism units.
They also include the so-called "Golden Division" special forces who answer directly to the prime minister. They are considered the best in the country but have had a torrid schedule on the frontline of every major battle against IS for close to a year.
The regular police and army forces have shown alarming weaknesses. Their battle-readiness is seen as badly below par and their commitment came into question again in Ramadi, when some defied orders and fled.
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- HASHED AL-SHAABI
The Popular Mobilisation (Hashed al-Shaabi) is an umbrella organisation for mostly Shiite fighters, including some well-established Iran-backed militias and volunteers.
It is nominally under the control of Prime Minister Haider al-Abadi but the powerful militias openly disagree with him.
Thousands of Shiite Hashed fighters have converged on the Ramadi area to take part in operations. Some of the militias have seasoned commando units with experience in Syria and elsewhere in Iraq.
They see the battle in the Sunni province as necessary to protect their own heartland and Shiite holy sites near the eastern border of Anbar and as a way of flexing their muscle.
- SUNNI TRIBESMEN
More than 1,000 of them have joined the Hashed's first unit of paid and vetted Sunni fighters. Other tribal fighters are fighting alongside the security forces as volunteers.
They are seen as the key to victory by the United States for their knowledge of the terrain and because the mass deployment of Shiite fighters in the area risks causing deep unease in the local population.
They are determined and seasoned fighters but have mostly light weapons and will need support from the government and the United States.
- US-LED COALITION
The coalition headed by the United States will take part in the battle from the air. It strikes IS targets in Anbar on a daily basis.
The air campaign is limited by the fact that civilians remain in Ramadi and quality of targeting is constrained by the lack of US troops on the ground.