That's the growing force of Iraqi Shiite fighters who're also fighting to keep Syrian President Bashar Assad in power.
Many of them were trained by Hezbollah and Iran's Islamic Revolutionary Guards Corps to fight the Americans in Iraq, and now form a major element in Iran's new "foreign legion," intelligence officers and military experts say.
A study by Matthew Levitt and Aaron Y. Zein of the Washington Institute for Near East Policy found that Hezbollah of Lebanon, Iran's most powerful ally in the Arab world, is "establishing local proxies in Syria ... through which it can maintain influence and conduct operations to undermine stability in the country in the future."
The study, "Hezbollah's Gambit in Syria," was published in August by the Combating Terrorism Center at West Point, the U.S. military academy's center for terrorism studies.
Western and Arab intelligence agencies, already worried about the ongoing rise of Sunni jihadist groups in Syria that are fighting Assad and his Shiite allies, are concerned that Iran's emergent foreign legion could be extended to militarize Shiites across the region and inflame the Sunni-Shiite religious war that is becoming the primary conflict in the Middle East.
Duplicating the partnership it had with the Revolutionary Guards' elite covert operations wing, the al-Quds Force, in Iraq, Hezbollah is using two groups it helped create in Iraq to train and equip several paramilitary Iraqi Shiite groups to back up Assad.
The two groups are Kataeb Hezbollah, or Hezbollah Battalions, and Asaib Ahl-Haq, or League of the Righteous, which were established by the al-Quds Force and Lebanon's Hezbollah in southern Iraq in the mid-2000s, primarily to attack U.S. forces.
Tehran called in Hezbollah, which drove the Israelis out of south Lebanon in May 2000 and fought Israel to a standstill in a 34-day war in 2006, to train Iraqi Shiites in 2006 and pass on the lessons it had learned battling the Israelis.
Hezbollah veterans also worked with and assisted other Iraqi Shiite groups now operating in Syria against rebel forces. These are Jaish al-Shabi, Liwa Abu Fadl al-Abbas, Kataeb Sayyed al-Shuhada, Liwa Zulfiqar and Liwa Ammar ibn Yassir.
Some of these have contributed fighters to the Syrian regime's National Defense Force, a supposedly 50,000-strong militia force of Iraqi Shiites and members of Syria's minority Alawite sect that dominates the Damascus regime. The NDF has become a key assault force with Assad's security apparatus.
Jaish al-Shabi -- the People's Army -- was designated a terrorist organization by the U.S. Treasury Department last December. It's now considered part of Damascus' powerful security apparatus, underlining "how the regime has adapted its forces to fight an asymmetric and irregular war" against the factionalized opposition, the study observed.
"In contrast to Jaish al-Shabi, the other militias are not within Syria's security apparatus, but are new independent proxies allegedly established with the assistance of the IRGC and Hezbollah," it said.
"Of the other four Iraqi militias, the Abu Fadl al-Abbas Brigade is the most prominent and has been involved in the conflict since the fall of 2012.
"The al-Abbas Brigade's fighters are a combination of members of Lebanese Hezbollah, Kataeb Hezbollah and Asaib Ahl al-Haq," or AAH, a force with a reputed strength of 2,000-3,000.
It operates mainly in southern Damascus, where Shiite fighters are protecting the Sayyida Zeinab shrine, the tomb of the Prophet Muhammad's granddaughter and one of Shia Islam's holiest sites.
The Abu Fadl al-Abbas Brigade is reported to have grown considerably in strength, possibly using Iranian gold to swell its ranks, in recent months.
"Its expansion marks a potentially dangerous turn for the region, giving Tehran a transnational Shiite militant legion that it could use to bolster its allies outside Syria," said Michel Knights, an Iraq expert at the Washington policy institute.
Kataeb Sayyed al-Shuhada is a 200-strong group established in April. It's led by Abu Mustafa al-Sheibani, aka Hamid al-Sheibani, an Iraqi Shiite warlord who has worked with the al-Quds Force since the late 1980s.
The al-Shuhada group and Liwa Zulfiqar, the Zulfiqar Brigade, also operate in south Damascus.