With at least 3,200 soldiers and officers from the Revolutionary Guards and other Shiite semi-official organizations, composed mainly of Afghan and Pakistani militants, Iran is second only to the Russian Federation in terms of engagement in the Syrian war to support Assad.
Moreover, Hezbollah - the Lebanese militant Shiite faction - is present in Syria with at least 4,500 soldiers and officers, but there are other Shiite groups, such as the People’s Mobilization Units (PMU), the former "popular defence brigades", operating in the Syrian region.
In all likelihood, it was Iran to persuade Russia to intervene in support of Assad, but the logic of Russia’s presence in the Syrian war is much more complex than it may appear at first glance.
In fact, the Russian Federation has placed the war against Daesh-Isis at the centre of its presence in the Syrian region, thus creating a new network of relations with the whole Arab world, including the one previously connected to the United States.
Russia made it clear it was necessary to eradicate the most immediate and severe danger for all Sunni Arab States, namely jihad, and this has led to its establishing new and effective relations with all those States.
Furthermore, Russia’s presence is a sign conveyed to Westerners that Syria's "cantonization" will never be accepted by the Assads’ Russian traditional ally because this would mean creating missile, terrorist, geoeconomic and naval positions that would directly undermine Russian interests in both the Mediterranean and the Greater Middle East, up to the Southern borders of the Federation.
Let us examine, however, the forces still operating in the Syrian war, including the smallest ones.
In addition to the friendly countries operating on the ground, support for Syria - including at military level - is provided by China, the Democratic People's Republic of Korea and the Polisario Front, also in clear contrast with Morocco, which indirectly supports - also through Saudi Arabia - the forces of the Syrian Democratic Army that is armed and supported mainly by the United States.
Besides the aforementioned Russia, Iran and Hezbollah, on the ground there are also some Palestinian groups and some Iraqi forces supporting Syria, especially with regard to intelligence and military activities on the border between Syria and Iraq.
Diplomatic support to Assad-led Syria is provided by Oman, Bolivia, Venezuela, Pakistan, Cuba, Zimbabwe, Belarus, Armenia, South Ossetia and Abkhazia.
The Syrian Alawite regime is also backed militarily and economically by Egypt, Algeria, Tunisia, Vietnam and India.
Russia has also sent to Syria some Chechen and Dagestan battalions as combat forces.
However, the opponents of the Assad regime - and hence of Russia, Iran and Hezbollah - include many groups of various origins, obviously all Sunni. Let us analyse them.
Jabhat al-Nusra, now called Jabhat Fateh al-Sham, is a network created by al-Qaeda in Iraq and Syria in 2011 - which became known in January 2012, during the possible Syrian "Arab Spring" - which also operates in the Lebanon, as well as in Syria.
Since its inception said movement was supported by Qatar and Turkey.
Harakat Ahrar al-Sham al-Islamiya is a coalition of jihadist groups supported by Saudi Arabia, Turkey, as well as Kuwait and Qatar.
Therefore, if Syria remains a Shiite Iran’s ally, at geoeconomic and political levels the strategic risk for the Gulf Emirates and for Saudi Arabia itself may become very high, especially in a phase of oil and financial crisis such as the current one.
Iran’s control over the Greater Middle East and the Persian Gulf would block any geopolitical autonomy of the Emirates and Saudi Arabia, with evident repercussions on the management of their oil resources.
The groups opposing Assad’s regime also include Asala wal-Tamiya, a coalition supported by Turkey, Saudi Arabia and the United States. Indeed, it was armed precisely by the United States and in the past it had operational links with Daesh-Isis.
Jabhat al-Shamiyah is an alliance of nineteen jihadist groups originating from the Muslim Brotherhood and this is exactly the reason why a Syrian ally like Egypt tacitly supports Assad's Alawite regime.
Jaysh al-Muhjahiddin is a further alliance of various Sunni guerrilla groups, all trained in Qatar, which in December 2016 merged with two other jihadist groups and later joined Ahrar al-Sham.
Therefore tribal equilibria, strategic and operational advantages, as well as interests of the funding countries, are at the origin of this multiplying and merging of militant jihadist groups.
Ajnad al-Sham is a typically Salafi group, always operating closely with Ahrar al-Sham.
Jaysh al-Islam, identified as terrorist organization by Russia, Egypt, Iran and Syria, is the second main pole of the Saudi indirect presence in Syria.
On the contrary, the groups siding with the Syrian Baathist government include Quwat Muqatili al-Ashair, a tribal force in which there is also a Druze contingent.
The list of these groups also include Liwa al-Jabal, consisting of five units originating from the Suwayda Governorate.
The pro-Assad forces count also Saraya al-Tuhid, the fully Druze force allied with Hezbollah, which was created in October 2016.
It is also worth recalling Labuat al-Jabal, the female Druze brigade created in July 2015.
Again in the Suwayda Governorate there is Qatib Jalamid Urman, patrolling mainly the border between Syria and Jordan. The Druzes operate also with Qatib Humat al-Diyar.
The Syrian Christians contribute to defend Assad’s regime with Asad al-Qarubim, a brigade created in 2013 after the attack on the Saidnaya Monastery.
There are other five Christian brigades, divided between Damascus, Homs and Quraytin, which were established to defend the Christian holy places in Syria and currently operate - together with Hezbollah and the Syrian Arab Army - throughout the Syrian national territory.
Conversely, Quwat al-Ghabab are the brigades created by the Greek-Orthodox communities and operate in Hama, Latakia and Tal Uthman.
The list of the groups siding with Assad’s regime also include Quwat Wad al-Sadiq, created in 2012 at the Sayyidah Zaynab Shiite Shrine near Damascus, which is connected with Hezbollah and composed of both Shiites and Druzes.
It is also worth recalling Liwa Muqtar al-Thiqfi, created in 2016 in memory of the ancient commander who attempted to avenge - against the Umayyads - the sacrifice of Imam Husseyn.
It operates on the Latakia front and is directly linked to Iran’s Revolutionary Guards.
Unlike the other smaller brigades, it is a force of approximately 5,000 units.
Again in the Latakia area, there is Saraya al-Arin, a Shiite group founded in 2015, while Liwa Sayf al-Mahdi is present in the Sayda Zaynab region, the centre of the traditional Shiite presence in Syria.
The group Liwa al-Imam Zayn Abidin, created in 2013, operates in Deir el Zur, the place at the core of the clash between what remains of Daesh and the Syrian regime, which has just been liberated by the “tigers”, namely the special forces of the Syrian Arab Army.
On the contrary, Liwa al-Jalil, the Galilean brigade founded in 2015, is a secular, leftist, nationalistic, Arab and pro-Palestinian organization.
The Syrian-Palestinians also operate within Quwat al-Jalil, created in 2011, while the "Leopards of Homs" (Fuhud Homs), a special operations regiment, operates in the desert areas around Homs and have also participated in the Syrian Arab Army’s actions in the Daraya region.
Also Liwa Qibar, established in 2013 and counting 4,000 units, is active in Homs.
It has also operated at Hama and al-Mansura.
Qatib al-Jabalui is an Alawite military structure operating in Homs, Dara, and in the Jazal areas.
Another of the many pro-Assad groups, namely Fuj Mughuyr al-Badiya, set up in 2015, has carried out its actions in the desert of Homs and Aleppo. It is connected to the Shaytat tribe that is active in Deir El Zur.
Also Liwa Asad al-Huseyn was created in 2015 and is mainly active in Latakia.
Liwa Dir al-Watan was founded in 2015 and designed with the specific aim of defending Damascus.
These are the main groups supporting Assad’ Syrian Arab Army, accounting for 50% of its forces.
This means that all the brigades listed here are worth 50% of the Syrian Arab Army.
Furthermore, in Syria, Hezbollah immediately divided into two groups: Jaysh al-Imam al-Mahdi, fighting mainly in the Tartus and Aleppo regions, and Quwat al-Ridha, operating in Damascus and in the neighbouring areas.
Both groups operate in close contact with Assad’s forces.
While Russia wages its war in Syria, Iran tightens the clamps on the Syrian forces.
In the Qalamun region there is also Quwat Dir al-Qalamun, namely people’s brigades trained by the Syrian Arab Army that control the Al-Hadath pipeline and participate in the clashes against jihadists between Aleppo and Nassiriya.
People’s brigades coordinated by the Syrian Air Force military intelligence operate also in Hama.
Finally, the Fifth Assault Corps is a counterinsurgency organization set up in 2012 within the Syrian Armed Forces with the fundamental support of Hezbollah and Iran.
It is present in nine Syrian provinces and supervises enlistments, as well as closely controlling Syria’s civil society.
Hence what does Iran want to obtain with its engagement in Syria?
Firstly, Iran obviously need to establish safe transit routes to logistically support Hezbollah in the Lebanon.
This is the real strategic danger for Israel, rather than the danger constituted by the Golan Heights, which have somehow already been made safe.
Secondly, an equally important Iranian goal is to closely monitor the Euphrates valley, which is rich in oil deposits that must not be acquired by the United States and its allies, still present north of the Euphrates.
With a view to accomplishing this strategic linkage, the Shiite Republic must transit through Iraq so as to reach Aleppo from Palmira.
Another Iranian route to penetrate the Syrian desert could start exactly from Deir El Zor and later expand into the Hasakah Province.
In fact, Iran has already sent over 3,000 Revolutionary Guards and People’s Mobilization Units (PMU), namely the Shiite paramilitary forces, to the area between Tanaf and Deir El Zor.
As to the other channel, considering that there are no significant Shiite, Druze or Alawite forces in the region, the Pasdaran are dealing directly with the Sunni tribes between Hasakah and Aleppo.
Russia, however, is backing the Iranian operations with its air forces.
Nevertheless Russia will not accept Iran’s gradual penetration of the Syrian State and military structures for a long period of time.
For the time being, precisely with a view to blocking Iran’s influence, the Russian Federation's proposal has been to quickly establish a Fifth Division of the Syrian Arab Army.
This would obviously serve to absorb - under the Syrian command - the tribal and territorial forces that could soon become pawns of the Iranian game in the Syrian desert.
Nevertheless - as is locally customary, and considering that the ongoing war has even enhanced these traditions - the various militias that have so far agreed to enter the Fifth Division have maintained their chain of command and their tactical and strategic autonomy.
Hence, Assad, is about to accept - de facto if not de iure - the Iranian droit de regard enabling it to control his territory and his armed forces.
Therefore, in the absence of a rational US strategy in Syria and vis-à-vis Iran, Russia thinks that the best thing to do - at least for the time being - is to support Iran in Syria and Iraq so as to exploit its potential against the United States and keep the Turkish ambitions on Western Syria under control.
This happens while the Kurds are turning into a pro-Western militia to control Turkish operations in Syria - in tacit agreement with Russia.
Moreover, the United States has already decided to defend the YPG Kurds (and, in the future, the PKK ones) only against the Turkish aims, while Iran and Russia will try to control all Syrian borders, including those with Turkey where US interposition forces are currently present.
Hence either the United States sends other troops to control Iran’s expansion within Syria - for the time being favoured by Russia - or the United States is bound to withdraw completely from the Syrian-Iraqi region.