Syrian Arab Army
The SAA is the national conscripted army of the Syrian Arab Republic. Much of its leadership and officer corps, such as Chief of Army Staff Gen. Ali Abdullah Ayyoub, hail from Alawite sect. As of April 2015, the SAA had an estimated 125,000 active personnel. Outside of the northwest and south, where the entirety of conflict with the opposition exists, the SAA is present in the besieged city of Deir ez-Zor and several pockets of government control in Hasakah government, where they appear to have de-facto peace agreement with the SDF.
National Defence Forces
A volunteer pro-government militia that was formed 1 November, 2012 in order to consolidate localized Popular Committee militias that had appeared. It creation was overseen by Iranian Quds Force commander Qasem Suleimani. Its members largely come Syria's ethnoreligious minorities: Alawites, Christians, Druze, and Armenians, and is approximately 100,000 strong. Members of the NDF receive salaries and equipment from the government.
Translating to the "Party of God," this is Lebanese Shia militant group and political party, led by Hassan Nasrallah, is renowned for its fighting capability. It entered the Syrian Civil War on the side of its longtime ally Bashar al-Assad in 2011 or early 2012, though did not admit involvement until the following year. Hezbollah has largely been active in areas close to the Lebanese border, from Qusayr to Zabadani, though has also been involved in other fronts, such as Aleppo. It has lost upwards of 2,000 fighters during its involvement. Hezbollah works in close coordination with military commanders of the regime, the Iranian Revolutionary Guard Corps and Iraqi Shi'a militias.
Iranian Revolutionary Guard Corps
The IRGC is the branch of the Iranian Armed Forces tasked with national security concerns. Prior to the Syrian Civil War, several thousand IRGC members were stationed in Syria, overseeing the training of Syrian troops, as well as the Iranian supply routes through Syria to its Lebanese ally Hezbollah. Iran has led what is known as the "Axis of Resistance," an alliance of itself, Syria, Hezbollah, and other actors, attempting to counter the influence of the West, Israel, and the Sunni Gulf states within the Middle East. In early 2012, IRGC deployment to Syria appeared to be escalating, in response to rebel success on the ground. On 16 September 2012, IRGC Commander General Mohammed Ali Jafari publicized the presence of the IRGC special forces branch the Quds Force in Syria, acting as advisors. Since then, Iranian presence has only increased. Quds Force commander General Qasem Soleimani has become the main figure behind Iranian intervention in the war, even flying to Moscow in July 2015 to help coordinate the joint Syrian-Iranian-Russian offensive to take place that fall. IRGC forces within Syria include the aforementioned Quds Force, the Basij paramilitary force, and multiple units of non-ethnic Iranian Shia militias, including Liwa Fatemiyoun, made up of Afghani immigrants to Iran. A similar Pakistani unit also exists, known as Liwa Zainabiyoun.
Russian Armed Forces
Since the beginning of the civil war, Russia has provided military aid and political support for its longtime ally Syria. However, on 30 September 2015 it announced that it was going to begin direct military involvement, at the behest of the Syrian government. This intervention has mostly taken the form of aerial bombardment, though Russian security contractors are known to operator on the groups. This campaign marks the first time the Russian Air Force has used its brand new Sukhoi SU-34 fighter-bombers, as well as its first operational use of precision-guided munitions. Also, on 7 October 2015, 26 of the brand new 3M-54E cruise missiles were launched from four Russian Navy warships in the Caspian Sea, travelling 1,500km to strike targets in Syria.
Other Local Militias
Though many local militias were incorporated into the National Defence Forces, independent ones still exist, formed on political and sectarian basis. Kataib al-Baath (logo shown here) is a volunteer militia of (mostly Sunni) Syrian Baath Party members in Aleppo and is approximately 5,000 strong. Other militias include Nussur al-Zaoubaa, a group of Syrian Social Nationalist Party members, the vaguely leftist but mostly Alawite and Shia sectarian group the Syrian Resistance, the Syriac Christian Sootoro militia in Hasakah governorate, and many more. These militias work as local security officers in regime controlled areas, as well as fight alongside the SAA on the frontlines.
Iraqi Shi'a Militias
In late 2012, Liwa Abu Fadl al-Abbas was created by Iraqi Shi'a for the purpose of protecting Shi'a shrines and religious sites within Syria. Soon after, multiple Shi'a militant groups from Iraq (created with Iranian support during the American occupation) announced their presence in Syria, including Kataib Hezbollah, Asa'ib Ahl al-Haq, Kataib Sayyid al-Shuhuda, and the Badr Organization. Most of these returned to Iraq in the winter of 2013, in order to counter the Islamic State offensive, but returned to Syria within the following year. These groups operate in conjunction with Hezbollah and Iranian forces, often being under their direct contol.