Most factions in the Syrian civil war received support from countries, groups, and entities based outside of Syria. As a result of such interventions, the war in Syria became a series of overlapping proxy conflicts, including between Russia vs the US,U.S. vs. Russia: What a war would look like between the world′s most fearsome militaries Militarytimes.com, 24 January 2016. Iran vs Saudi Arabia, Iran vs Israel, Israel vs Turkey, and Russia vs Turkey. The war led to spillover fighting in countries including Lebanon, Iraq, and Libya. The final years of the war also served as a part of the Middle Eastern crisis.

The Syrian government was supported by Russia and Iran: both intervened militarily. Iranian proxies including the Lebanese paramilitary group Hezbollah as well as Palestinian and Iraqi militias provided additional military presence. Iraq intervened to support Assad, mostly fighting the Islamic State.

Russia became embroiled in clashes and diplomatic incidents with Turkish, American, and Israeli forces, while Iran engaged in direct conflict with Israel. Egypt supported Assad.

The various factions that made up the Syrian opposition had wider international support, receiving financial, political and military aid from Middle Eastern powers (Turkey, Saudi Arabia, Qatar, the UAE, Jordan, Israel) and Western countries (US, United Kingdom, France, Germany, Netherlands). Beginning in 2016, Turkey began a series of cross-border operations to support the Syrian National Army. The US, UK, and France supported the Revolutionary Commando Army, with troops stationed in Al-Tanf while also attacking the Syrian government. Support for the Syrian opposition waned following 2017 as a result of increasing internal divisions and the defeat of the Islamic State. Turkey was the main supporter of the opposition, sponsoring the campaign that led to the fall of the Assad regime; Ukraine supported the opposition due to the Russo–Ukrainian war. Israel attacked Iranian and Syrian government forces throughout the war, while repeatedly denying supporting the opposition.

The Syrian Democratic Forces (SDF) of Rojava mainly received military and financial support from the American-led coalition fighting the Islamic State. The SDF received military support from Kurdish militant groups based in Kurdistan, as well as by left-wing foreign fighters. From 2015 onward, Turkey began to attack the SDF as a result of the Kurdish–Turkish conflict, leading to the Turkish occupation of northern Syria and for Iran and Russia to provide limited support to the SDF in order to prevent opposition territorial gains. Israel offered diplomatic support to Rojava in an effort to counter Iranian influence. The US remained the SDF's foremost sponsor.

Support for the Assad government

Russia

Russia became a military ally of Syria in 1956, and continued supplying Syria's government with arms, sending military and technical advisers to train Syrian soldiers, and helping to repair and maintain weapons. Investigations suggest that Russia transported hundreds of tonnes of banknotes into the country.

In December 2012, it was reported that Russian military personnel were inside Syria, manning anti-aircraft systems."Russian military presence in Syria poses challenge to US-led intervention" . The Guardian, 23 December 2012. Retrieved 26 February 2013. The depth and sophistication of Syria's air defences was cited as a reason for the US not to intervene militarily or impose a no-fly zone, despite President Obama's commitment to do so if the Assad government crossed the "red line" of using chemical weapons.

Western leaders repeatedly criticized Russia's support of the Syrian government; Russia claimed that its actions had not violated international law. In June 2012, Russian president Vladimir Putin said Russia did not support "any side" in the conflict.

In December 2013, Russia was reported to have supplied armored vehicles, surveillance equipment, radars, electronic warfare systems, spare parts, and weapons such as guided bombs.

On 30 September 2015, Russia conducted air strikes against ISIS, the Al-Nusra Front, and other perceived enemies. The Russian Orthodox Church official spokesman called the intervention a "holy fight" (or holy struggle) against terrorism. Russia claimed the attacks were against ISIL. However, Russian air strikes also targeted positions held by the Army of Conquest coalition, including Saudi/Turkish-backed Al-Nusra Front and by ash-Sham.

In autumn 2015, the US ruled out military cooperation with Russia. On 20 October 2015, the US and Russia signed a secret technical memorandum of understanding to avoid air incidents.U.S., Russia sign memo to avoid clashes in air over Syria Reuters, 20 October 2015.

On 22 November 2015, Assad said that with two months of air strikes, Russia had achieved more than the US-led coalition had achieved in a year. Two days later, Obama said, "Russia right now is a coalition of two, Iran and Russia, supporting Assad. Given Russia’s military capabilities and given the influence they have on the Assad regime, them cooperating would be enormously helpful in bringing about a resolution of the civil war, and allow us all to refocus our attention on ISIL."

As of Summer 2023, Russia had 20 military bases in Syria as well as 85 other military outposts, the majority in Hama, Al-Hasakah, Latakia and Aleppo. The main bases were Hmeimim Air Base in Latakia, designed for vertical take-off and landing operations, and used for military operations since 2015. Tartus Naval Base was Russia's only Mediterranean naval base. In March 2023, Assad told Russian media that "increasing the number of Russian military bases in Syrian territory might be necessary in the future because Russia's presence is linked to the global balance of power."

In January 2025, the Syrian caretaker government ended the treaty allowing Russian military presence in Syria, including a contract with Russian company Stroytransgaz to manage the commercial areas of the Tartus port. In February, Syrian defence minister Murhaf Abu Qasra stated that Russia would be allowed to maintain the base "if we get benefits for Syria out of this". By October, Russia was using the port for resupplying its airbase while negotiations over its fate continued.

Iran

Iran and Syria were strategic allies. Iran provided significant support. This included technical support, combat troops, and $9bn. Iran viewed the civil war as a critical front in an existential battle that directly affected its geopolitical security. Iran's supreme leader, Ali Khamenei, was reported in September 2011 to vocally support the Syrian government. Zabadani was important to Assad and to Iran because, as late as June 2011, the city served as the Islamic Revolutionary Guard Corps's (IRGC) logistical hub for supplying Hezbollah.

In the civil uprising phase of the war, Iran provided Syria with technical support. As the uprising expanded, reports of Iranian military support increased, partly in response to reports of increasing opposition support from Persian Gulf states. On 30 January 2013, ~10 jets bombed a convoy believed to be carrying Russian-made SA-17 anti-aircraft missiles to Lebanon. The attack, attributed by media reports to the Israeli Air Force, did not result in counterattacks from Syria, who reserved the right to retaliate. Western intelligence sources reported that Iranian general Hassan Shateri was killed in the airstrike. Iran acknowledged his death without details. Israel did not comment on its involvement.

In autumn 2015, Iran signed the road map based on the 2012 Geneva Communique that was worked out during two rounds of talks in Vienna. After a meeting between Vladimir Putin and Ali Khamenei in Tehran on 23 November 2015, Iran was said to have decided to match Russia's stance vis-a-vis the Syrian leadership.

In 2024, Iran maintained 55 military bases in Syria and 515 other military points, the majority in Aleppo and Deir Ezzor and the Damascus suburbs; these comprised 70% of the foreign military sites in the country.

Hezbollah

Hezbollah was a longtime ally of the Ba'ath Party and helped the Syrian government fight the opposition. As early as November 2011, protesters burnt Hezbollah flags and images of its leader Hassan Nasrallah, while pro-government protesters carried posters of him.

In August 2012, the US sanctioned Hezbollah for its alleged role in the war. Nasralla denied fighting on behalf of the government, stating in a 12 October 2012 speech that "right from the start the Syrian opposition has been telling the media that Hezbollah sent 3,000 fighters to Syria, which we have denied". However, he said that Hezbollah fighters had gone to Syria independently and died there doing their "jihadist duties". Hezbollah stated it supports a reform process and is against what it called US plots.

In January–February 2012, Hezbollah fighters were reported to have helped the government fight in Damascus and in the Battle of Zabadani. Later that year, Hezbollah fighters took over eight villages in the Al-Qusayr District."Hezbollah fighters, Syrian rebels killed in border fighting" . Al Arabiya, 17 February 2013. Retrieved 18 February 2013. Nasrallah said that Hezbollah fighters helped the government maintain control of ~23 Lebanese Shiite villages. In September 2012, Hezbollah's commander in Syria, Ali Hussein Nassif, was killed along with other Hezbollah militants in an ambush by the Free Syrian Army (FSA ) near Al-Qusayr.

According to the US, the Assad loyalist militia known as Jaysh al-Sha'bi was created and maintained by Hezbollah and IRGC, providing money, weapons, training and advice. Also, according to Israeli intelligence sources, Hezbollah worked to forge loyalist militias into a 100,000-strong irregular army. The American-Israeli security relationship: Let’s try a less awkward embrace