Jihadists, peshmerga, Shiite militia, anti-Assad rebels, regular troops and foreign advisors, a short summary of who is fighting who in Iraq and Syria:
Sunni Arabs, they are fighting in Syria against a regime dominated by Alawites, a sect rooted in Shiite Islam and in Iraq against the central government.
The CIA has estimated the number of jihadists in both Iraq and Syria at between 20,000 and 31,500, three times its previous number. Most belong to the Islamic State (IS) organisation.
Foreign fighters in Syria are believed to number 15,000, of whom some 2,000 come from Western countries, according to the CIA. Some have joined the IS.
The London-based International Center for the Study of Radicalisation (ICSR) says foreign jihadists in Syria come from 74 countries, mainly in the Middle East and North Africa.
A breakdown of leading source countries puts Tunisia on top with up to 3,000 fighters, followed by Saudi Arabia with up to 2,500, Morocco and Jordan with 1,500 each, and Lebanon with 900. About 800 are thought to have come from Russia, and 700 from France.
Since June, they have been on the front line against jihadists in Iraq. Known as peshmerga, or "those who confront death" in Kurdish, they come from an autonomous Kurdish region of northern Iraq and are allied with Washington. They number around 200,000.
In August, around a dozen countries, including Britain, France, Germany and the US started weapons deliveries to Kurdish forces.
Kurdish leader Massud Barzani said on August 26 Iran was the first to do so.
Kurdish forces also received backing from affiliated groups in Syria and Turkey last month, in particular from the Kurdistan Workers Party (PKK), which Turkey, the EU and the US consider a terrorist organisation.
Iraqi Army, Shiite Militia And Foreign Advisors In Iraq:
Iraq's armed forces are being reshaped by the US and its allies and number 271,400 troops, of which 193,400 belong to the army, according to International Institute for Strategic Studies (IISS).
The army failed to resist a drive by IS fighters in June and has been unable to check rising violence since US troops withdrew from Iraq in late 2011.
Shiite religious leaders have urged their followers to take up arms to defend the country. Moqtada al-Sadr, one-time head of the now disbanded Mahdi Army, has since created the Saraya al-Salam, or Peace Brigades.
US President Barack Obama sent US troops to Iraq in June to train Iraqi forces, where they now number around 1,600, with the addition of another 475 military advisors announced on September 10.
Canada said on September 5 that it would send several dozen advisors as well.
Syrian Army, Pro-Government Militia:
Syria's armed forces are made up of 178,000 troops, of which the army accounts for 110,000, according to the IISS. The civil war that began in 2011 has halved its numbers but it has since been restructured.
The army is backed by militia largely drawn from Syrian President Bashar al-Assad's own Alawite community.
The Syrian regime is also supported by fighters from the powerful Lebanese Shiite movement Hezbollah, and the IISS says Iran provides "considerable" financial and military aid.
Many groups in addition to the IS organisation are fighting the regime of Syrian President Bashar al-Assad, in particular Islamists from the Al-Nusra Front, which is affiliated with Al-Qaeda, and Ahrar al-Sham.
Estimates of their total number run to around 100,000 and include fighters from the moderate Free Syrian Army, who probably total no more than 15,000.