BEIRUT (Reuters) - Syria's rebel movement has been a constantly shifting array of groups and alliances since the uprising against President Bashar al-Assad began nearly three years ago.
Assad's security crackdown transformed Syria's largely peaceful protest movement in March 2011 into an armed insurgency in the first year of the revolt, and since then opposition formations have been increasingly overtaken by Islamist groups.
As new leaders have emerged within the opposition, infighting intensified and reached a new level this month, with several rebel factions declaring war against the radical Islamic State of Iraq and the Levant (ISIL).
Below is a description of some of Syria's main rebel groups:
An amalgam of six major Islamist groups, this alliance is believed to be the biggest rebel army working in Syria. Its formation last November gutted the Western-backed Syrian Military Council, depriving it of some of its main members, such as the Tawheed Brigade, and further distanced it from powerful Islamist groups like the Ahrar al-Sham Brigades.
The Islamic Front's members are hardline Sunni Islamists who want Syria to become an Islamic state, but they have been more tolerant of other groups than the radical al Qaeda branch, the Islamic State of Iraq and the Levant.
Analysts say the number of fighters brought together by the Front is between 40,000 to 50,000. It is still not clear, however, whether it will be more successful in coordinating and leading Syria's notoriously fractious rebel groups compared to the failed moderate opposition alliances.
The Islamic Front has not formally declared war on ISIL and its attitude towards the group is still ambiguous, but many of its leading factions are participating in the attack on ISIL.
* Syrian Revolutionaries Front:
This alliance of largely non-ideological rebel units was formed in December and helped launch a growing campaign against hardline ISIL fighters.
The backbone of the group is the Syrian Martyrs Brigade, a once powerful group from the northern province of Idlib led by Jamal Maarouf. Maarouf and his fighters were largely discredited in Idlib by rival Islamist groups who accused them of diverting funds meant for the front lines into their own pockets.
Unlike most other rebel formations, the group does not appear to have strong ideological leanings, though its units are mostly moderate Islamists.
The SRF is believed to receive funding from large Gulf countries like Saudi Arabia, given that Riyadh was said to be Maarouf's main financier. It has poor relations with the Islamic Front but has expressed support for the Western and Gulf-backed Supreme Military Command (SMC), the foundering successor to the leadership of the failed Free Syrian Army (FSA).
The FSA was the original umbrella group for the rebels but was never able to form a coherent organizational structure or leadership and the SMC has faced similar challenges.
Some analysts suggest that the SRF may be another attempt at reviving the main components of the FSA, but it still lacks the regional scope to try that, as most of its member units hail from the north.
* Mujahideen Army:
This recent formation of eight Syrian militant groups was announced early in January and almost immediately launched a campaign against ISIL, leading many observers to believe it may have been formed by Gulf Arab backers for the purpose of challenging ISIL.
The group, which claims to have 5,000 members, is seen as moderately Islamist. Most of the factions that joined the Mujahideen Army are relatively minor and little is known about the group so far.
However, this new group, along with the Syrian Revolutionaries' Front, spearheaded the campaign against ISIL that has broken out in many opposition-held parts of northern and eastern Syria.
* Nusra Front
This powerful rebel group is comprised of both Syrians and foreign militants and has been formally recognized by the central leadership of al Qaeda as its franchise in Syria.
The group was one of the first to use techniques such as suicide attacks and car bombings in urban areas. Despite this, it is seen as more tolerant and less heavy handed in its dealings with civilians and other rebel groups in comparison with its rival al Qaeda affiliate, ISIL.
Nusra Front, estimated at around 7,000 to 8,000 members, has worked with most rebel factions fighting in Syria but follows an austere version of Islam and calls for the creation of an Islamic state.
It is not formally a part of the Islamic Front but it works closely with many member groups. Some of its units have joined in recent rebel-on-rebel battles against ISIL but it has not officially declared war on the group.
The Nusra Front's leader, known as Abu Mohammed al-Golani, has called for a ceasefire between ISIL and other rebel groups but the move has done little to slow the fighting.
* Islamic State in Iraq and the Levant:
ISIL was formed by breakaway elements from the Nusra Front, al Qaeda's official affiliate in Syria, who joined with al Qaeda's Iraq branch.
The group is headed by the Iraq branch's leader, known as Abu Bakr al-Baghdadi. He has ignored calls from al Qaeda central to stay out of Syria and focus on Iraq.
ISIL is seen as the most hardline of all the Islamist forces in Syria and has made enemies of several rebel groups since it seized control of many towns and checkpoints in opposition areas.
The group was largely accepted by Syrian civilians at first due to its strict policies against looting and its attempts to provide social services. It lost favor as its members began kidnapping and killing critics and rival groups.
ISIL is now fighting on several fronts. In Syria, many rebel factions are trying to retake territory and force the group out of their areas. At the same time, Iraqi military forces have launched a heavy campaign in Anbar province, where ISIL fighters took control of some towns.
While its numbers may be smaller, perhaps around 6,000 to 7,000, the ISIL's hardline fighting force is very committed and capable of surviving as the two countries in which it operates face chaotic sectarian conflict.
The group has vowed to use assassinations and other strategies to retaliate against attacks. In a January 7 statement, it vowed to crush the Syrian rebels and made no gestures toward reconciliation despite Nusra calls for a truce.
* Supreme Military Command
The SMC is a moderate, non-ideological group. It enjoys backing from Western powers such as the United States, as well as Turkey and Gulf Arab countries, and has never been able to shake the impression among local rebel groups that it was a leadership coming from abroad.
Many of its commanders spent much of their time outside the country. They were also unable to secure consistent supplies of arms or funding from foreign donors.
While still functioning nominally, the SMC was dealt a heavy blow by the formation of the Islamic Front in November 2013, which deprived it of some of its largest members and allies and further damaged the SMC's legitimacy.
(Compiled by Erika Solomon; Editing by Giles Elgood)