Mouhannad Droubi was a member of the Free Syrian Army, a collective of rebel groups in Syria. Reuters/Yousef Homs

Syria’s version of Arab Spring unrest began in March 2011 as activist-led, largely secular demonstrations against the brutal regime of President Bashar Assad. Four years later, with a rebellion turned into a civil war, those secular moderates are almost gone -- and the Syrian war has become anything but Syrian. Regional sectarian ties have turned Syria into an international battleground rife with fighters, funding and weapons from around the world.

Citizens of countries ranging from Canada to China have left their home nations to join factions fighting in Syria, like the Free Syrian Army or the Kurdish People’s Protection Units, terrorist organizations such as the Islamic State group and even the Assad regime itself. Most foreign fighters in Syria now belong to either Iran-backed, pro-Assad militias or the militant group formerly known as ISIL or ISIS. Together, Assad and the Islamic State group are responsible for the bulk of the more than 200,000 people killed since the war began, nearly all of them Syrian.

In the first years, the Syrian Civil War was being fought by a secular coalition of Free Syrian Army brigades against Assad’s armed forces, which were backed by the volunteer National Defense Force consisting largely of Alawites, the sect of Shiite Muslims to which Assad belongs. As the war dragged on, however, Assad’s ground forces shrank -- through casualties, desertions and draft dodging -- to an estimated 150,000 from 315,000 troops, which is forcing the regime to rely on irregulars such as the National Defense Force, local militias and foreign fighters.

Those units are now largely composed of Shiite groups acting as Iranian proxies, mostly from Iran-backed Shiite militias based in Iraq, and from Hezbollah, the Lebanese Shiite organization. Iran’s Islamic Revolutionary Guard Corps has mobilized roughly 70,000 fighters in Syria since the war began, Iranian officials said this year.

“You don’t see very many pure Syrian army formations anymore,” said Jeff White, a defense fellow at the Washington Institute for Near East Policy in the District of Columbia. “This is what basically kept the regime in the war.”

Iranian infiltration into Assad’s forces began in late 2012, when the Free Syrian Army was making major gains, thanks to the support of U.S. military advisers. Realizing the danger of losing a major ally, Tehran sent advisers from major Iraqi Shiite militias, Hezbollah and even the Islamic Revolutionary Guard Corps to aid Assad’s failing army.

“Major involvement by foreign forces on the regime side began in spring of 2013. Prior to this date, Lebanese Hezbollah and Iranian advisers had been involved in the formation and training of the Syrian National Defense Force,” said Christopher Kozak, a Syria analyst at the Institute for the Study of War in Washington. “The size and scope of the role played by foreign fighters has expanded ever since.”

A year later, Hezbollah’s role in the war had grown exponentially. When major fighting broke out among the Nusra Front (an affiliate of al Qaeda), the Free Syrian Army and Assad’s forces in Yabroud in early 2014, the regime had almost no Syrian fighters on the ground. Hezbollah was commanding the battle and only allowed the Syrian regime to participate through aerial bombardments and artillery support, according to a report by the Syrian Network for Human Rights.

High casualty numbers and the increasing reliance on nongovernmental militias make it difficult to tally the number of pro-Assad foreign fighters in Syria today. Most come from Iraqi militias and Hezbollah forces, but recently Afghan, Pakistani and some Yemeni volunteers have been fighting along with the regime, the Syrian Network for Human Rights reported. There have also been accounts of Americans and Europeans affiliated with Hezbollah going to fight on the Assad side.

By mid-2013, there were estimates that thousands of Iranian Revolutionary Guard fighters were playing combat roles in Syria. The most widely circulated estimate for Hezbollah fighters in Syria is about 5,000 at any given time, but some sources have raised the estimate to almost 10,000 since the Islamic State group seized parts of Iraq, forcing many Iraqi militias to return home to fight. That has left open a space being filled by Hezbollah.

There are also dozens of Iran-backed Shiite militias operating in Syria today, with thousands of fighters. Most brigades are mixed, but Afghan fighters, Hezbollah affiliates and even a contingent of Saudi Shiites have been present, taking direct orders from Iran.

Their enemies, Sunni organizations such as the Islamic State group, have also been major players in attracting foreign fighters to the Syrian Civil War. In May 2014, a fighter with the Nusra Front, aka Jabhat al-Nusra, Moner Mohammad Abu-Salha, 22, drove 16 tons of explosives into a Syrian army base in Idlib province and detonated it, killing dozens and himself. Abu-Salha was an American citizen who resided in Florida.

Abu-Salha represents more of a growing trend than an isolated incident. Roughly 20,000 foreign fighters from more than 40 countries have traveled to join terrorist organizations such as the Islamic State group and the Nusra Front. Most come from Middle Eastern and North African countries, with particularly large contingents from Morocco and Tunisia. But hundreds of French, British and Belgian nationals have also ventured to Syria, among the estimated 3,400 Westerners who have joined the fight with the Islamic State group or similar organizations in Iraq and Syria, U.S. officials have estimated. In November, that number was only 2,700.

Despite the increasing infiltration of foreign fighters, most fighters -- and victims -- on the ground are still Syrian. But the increasing influence of foreign powers in Syria has overshadowed the reason thousands of Syrians took to the streets in 2011.

The toll of this deadly foreign influence is not lost on Syrians caught between the two sides that foreigners support.

“Assad is worse than ISIS, but we wish to get rid of both of them,” said an activist from Deir el-Zour who uses the name Abdul Hamid. “Then we will need someone to get rid of Nusra.”