Three and a half years into the Syrian civil war, it is clear that any hopes that the overall insurgency against the Assad regime might develop in a more "mainstream" and "moderate" direction have dissipated.
Hopes had been pinned on the rebel groups under the Free Syrian Army (FSA) banner in the southernmost province of Deraa (the "Southern Front"). Many of these groups, which espouse a vague, superficial religious identity, were the intended recipients of arms shipments approved by the United States in coordination with Saudi Arabia.
My focus is on the jihadi organizations the Islamic State (IS, formerly the Islamic State in Iraq and ash-Sham), the Syrian al-Qa'ida affiliate Jabhat al-Nusra, and an assortment of other groups in Syria that espouse the project of a transnational Caliphate and whose influence vastly overshadows the Southern Front.
Scope of IS control
The IS (by far the most influential of the extremist groups) now controls a vast expanse of contiguous territory in Syria that encompasses all the major urban localities of Raqqa province and Deir az-Zor province (with the exception of the remnant regime-held districts of Hawiqa and Rashdiya in Deir az-Zor city), as well as all territory in Hasakah province not controlled by the regime or the Kurdish autonomous Democratic Union Party (PYD). In addition, IS controls all major localities in eastern Aleppo province (Jarabulus, Manbij, al-Bab, Deir Hafer, and Maskanah) and is currently pushing westward in an attempt to seize more border areas with Turkey and reclaim its one-time border stronghold of Azaz, from which it strategically withdrew at the end of February 2014.
Explaining the Success of IS
The IS (when it was still known as ISIS) had been allowed to grow in strength, both in terms of manpower and weaponry, for a long time in the absence of a united rebel front against it; the organization was able to regroup in the face of a multipronged attack and to learn from its strategy mistakes (e.g., spreading itself too thin to be defensible).
Why didn't opposition groups fight ISIS in a concerted effort within the first six months or so of its existence? Some rebels feared the group was too strong, others believed that the focus should be to bring down the regime first, and still others (particularly from the Islamic Front faction Ahrar ash-Sham) wanted to work with ISIS on the basis of common ideological overlap and program. Even when infighting broke out in a number of localities, Ahrar ash-Sham affiliates either refused to fight against ISIS or were subsumed by it (e.g., throughout Hasakah province, even as the two had worked closely together fighting against the Kurdish militia Popular Protection Unit).
Evidence of divisions and disunity within rebel ranks came to light during the prolonged ISIS offensive deep into Deir az-Zor province; some groups (e.g., Abdullah ibn Zubayr Battalions) announced that they would not fight ISIS, even though the ISIS offensive reversed a rebel offensive to take the strategic southern Hasakah locality of al-Markadah. Other rebels of a variety of orientations had already defected, seeing ISIS as the "winning horse" on which they could safeguard their own interests; IS ranks grew dramatically over the past month or so as control over Deir az-Zor province has been consolidated.
The success of IS can be attributed in part to its use of various approaches. It uses its financial resources to engage in social outreach, with particular focus on 'da'wah' meetings and distributions of basic commodities like bread. It takes a hardline approach against criminality, thereby conveying a sense of order that has often not been delivered by other rebel groups but also crushing any opposition with public displays of brutality that have had a deterrent effect, whether through executions of individuals (and then publicly hanging them on crosses) or large scale massacres as meted out to rebellious members of the Sha'itat tribes in Deir az-Zor province.
Other Jihadi Groups
As territory is increasingly gobbled up by IS and the regime, other jihadi groups have formed their own proto-state/proto-state legislation projects, presenting challenges to nonextremist rebels in a "fight for what remains." The most controversial of these initiatives is the leaked "emirate" project announcement by Jabhat al-Nusra's leader Abu Muhammad al-Jowlani in July: in global jihadist thinking, the emirate is a country-specific or regional project as the stepping stone to a wider Caliphate. Although the group has officially denied the establishment of an emirate for the moment, its record on the ground suggests a desire to secure contiguous territory and strongholds from one-time allies—most notably the Syrian Revolutionaries Front (SRF) in Idlib, whose leader Jamal Ma'arouf had previously declared that he would fight IS but not Jabhat al-Nusra.
Now, however, Jamal Ma'arouf has begun to liken Jabhat al-Nusra's conduct to that of IS. The group's seizure of towns in Idlib province from the SRF might be prompted in part by the strategic need to control northern border movement of weapons and goods to FSA-banner groups, which are provided on condition of no cooperation with Jabhat al-Nusra. However, it is also clear that in the newly seized territories, Jabhat al-Nusra has imposed extensive social and economic regulations that point to proto-state building in keeping with the emirate project, contrasting with the group's previous strategy in Deir az-Zor province of building up support gradually through pragmatism and cooperation with locals and other rebel groups but focusing on control over local Shari'a committees.  Yet despite these tensions in Idlib, the SRF and the Southern Front continue to work with Jabhat al-Nusra in the south, which has allowed Jabhat al-Nusra to entrench itself firmly in Deraa and Quneitra governorates. This localization dynamic is similar to the rebel disunity in the face of the rise of ISIS and in the long run enables Jabhat al-Nusra to build influence on aggregate.
In July other jihadi groups also announced their own coalitions or intentions to impose Islamic law. The most notable of these is the Jabhat Ansar al-Din coalition, comprising Harakat Sham al-Islam (a group founded by Moroccan ex-Guantanamo detainee Ibrahim bin Shakaran who was killed in the second rebel offensive on Latakia in the spring), the Green Battalion (a group founded by Saudi fighters who wanted to stay out of the Jabhat al-Nusra-IS dispute), Harakat Fajr al-Sham al-Islamiya (a Syrian group that is pro-Caliphate), and Jaysh al-Muhajireen wa al-Ansar (ex-IS front group under Omar al-Shishani, now bound clearly to the Caucasus Emirate under Salah al-Din al-Shishani).
Jabhat Ansar al-Din released a manifesto on 13 August, declaring that one of its key aims is the establishment of "the rule of the law of God," while not ruling out cooperation with "honest factions." One wonders why the group did not simply subsume itself under Jabhat al-Nusra's wing in the manner of Suqur al-Izz. It is also notable that the Jabhat Ansar al-Din coalition received support from Jamaat Ansar al-Islam, a jihadi group originating from Iraq that has expanded into Syria and does not regard itself as part of al-Qa'ida. Jamaat Ansar al-Islam claims cooperation with Jabhat al-Nusra, but evidence suggests it is closer to Jabhat Ansar al-Din and Jaysh al-Muhajireen wa al-Ansar in particular.
While no public statements have been made to indicate tension between Jamaat Ansar al-Islam and Jabhat al-Nusra, the muhajireen group Jaysh Muhammad in Bilad al-Sham (primarily based in Aleppo province) announced in July its intentions for implementation of Shari'a and subsequently appears to have experienced a rift with Jabhat al-Nusra. Jaysh Muhammad had retained a presence in the Azaz area even after the withdrawal of IS and return of Northern Storm (now part of the Islamic Front) to govern the town. Northern Storm recently asked Jaysh Muhammad to leave Azaz, which has now culminated in a full withdrawal of the group as its leader headed off to Latakia.
The rapid growth of IS in Syria over the past few months has instead helped foster further divisions as extremists bring forth their true agendas that are ultimately not amenable to power sharing and compromise. The biggest losers in the ensuing scramble for power and influence are undoubtedly FSA-banner rebels, particularly those like the SRF who trusted in Jabhat al-Nusra as a fellow native Syrian brand. It is a safe bet that extremists—IS or otherwise—are set to be a part of the Syrian landscape for the long-term.
Terms like "moderate" (and its opposite, "extremist") are always problematic when it comes to analysis of Syria, often tossed about carelessly in partisan debate. Nonetheless, few would disagree that the Islamic State (IS; formerly Islamic State in Iraq and ash-Sham, or ISIS), the Syrian al-Qa'ida affiliate Jabhat al-Nusra, and an assortment of other jihadi groups in Syria espousing the project of a transnational Caliphate constitute extremist organizations.
 A social media representative for the Southern Front's Liwa al-Muhajireen wa al-Ansar described the group's ideology as follows: "Our religion is Islam and we came out demanding freedom … we are against extremism and we must bring down the regime and the people must live in security." Interview on 27 July 2014.
 For example, "Liwa al-Muhajireen wa al-Ansar: Targeting of the Katiba al-Dababat with a MILAN missile and some of the clashes," Liwa al-Muhajireen wa al-Ansar, 17 July 2014. The MILAN missile is an anti-tank missile. The fighting takes place in Sheikh Sa'ad in Deraa province. https://www.youtube.com/watch?v=NSTU7fecDI8&feature=youtu.be
 Abu Khalid al-Suri, a longtime veteran of global jihad who led the al-Qa'ida-aligned contingent of Ahrar ash-Sham, was appointed by Zawahiri to mediate the dispute between what was then ISIS and Jabhat al-Nusra in 2013. He was subsequently killed in February 2014 in a suicide bombing whose exact perpetrator still remains unknown. IS denied responsibility, but I believe that the operation was likely the work of IS and was intended to target the Ahrar ash-Sham base, though not necessarily to assassinate Abu Khalid al-Suri. The Saudi Sheikh Muheisseni tried to promote unity among jihadis; he organized joint da'wah events in Aleppo between ISIS, Jabhat al-Nusra, and Ahrar ash-Sham.
 One of the most well-known early cases is the November 2013 case of Saddam al-Jamal, who was originally commander of the Western-backed Supreme Military Council's local affiliate in the border town of Albukamal: Liwa Allahu Akbar. His emergence as a defector to ISIS was contrary to initial thoughts, including my own, that he had been abducted thus and his 'testimony' was made under duress.
 Gaining a monopoly on bread control has always been a key aspect of IS and its prior incarnations' strategies to increase power and influence in a locality where there are rivals to contend with.
 The idea of 'IS bringing order' is what first brought the group into some localities like Atarib in Aleppo province last year, as no one would deal with Hasan Jazrah's gang Ghuraba al-Sham. For a more recent case, see Goha's Nail, "Manbij and the Islamic State's Public Administration," 22 August 2014,http://gohasnail.wordpress.com/2014/08/22/manbij-and-the-islamic-states-...
 Jabhat al-Nusra, "A Clarification Regarding the Alleged Announcement of of an Emirate by Jabhat al-Nusra," 13 July 2014, http://justpaste.it/g7hh
 "I am not fighting against al-Qa'ida … it's not our problem," says West's last hope in Syria, The Independent, 2 April 2014,http://www.independent.co.uk/news/world/middle-east/i-am-not-fighting-ag...
 "Jamal Ma'arouf: the activities of Jabhat al-Nusra have begun to resemble the actions of the 'Islamic State' organization," Smart Press, 18 August 2014,http://justpaste.it/jamalmarufnusra
 "Jabhat al-Nusra announces its sets of laws in Idlib," All4Syria, 6 August 2014,http://www.all4syria.info/Archive/160651
 Jabhat Ansar al-Din manifesto, 13 August 2014,https://azelin.files.wordpress.com/2014/08/jabhat-ane1b9a3c481r-al-dc4ab...
 "Jamaat Ansar al-Islam in Bilad al-Sham: Blessing Jabhat Ansar al-Din (Analysis and Translation), Aymenn Jawad Al-Tamimi, 31 July 2014,http://www.aymennjawad.org/2014/07/jamaat-ansar-al-islam-in-bilad-al-sha...
 AnsarulSham, ask.fm, http://ask.fm/ansarulsham
 See for example "Prayers by the brothers of Jaysh al-Muhajireen wa al-Ansar and Jamaat Ansar al-Islam in Bilad al-Sham: Jamai'a Zahara', Aleppo, Syria," AnsarulSham, May 2014, https://twitter.com/althabaat_ansar/status/465980836954009600
 "Muhajireen Battalions in Syria: Part IV," Aymenn Jawad Al-Tamimi, 19 August 2014,http://www.aymennjawad.org/15207/muhajireen-battalions-in-syria-part-iv
 Conversations with Northern Storm spokesman Abd al-Qadir Abu Yusuf, 23 and 31 August 2014. This follows on from an official Islamic Front request- in polite 'brotherology' terms- to Jaysh Muhammad to evacuate Azaz. See ibid. for this. In reality, the tensions are much greater than the Islamic Front's statement lets on. The Northern Storm spokesman claimed to me that the rift between Jabhat al-Nusra and Jaysh Muhammad was caused by the latter's refusal to fight IS.