Terrorist groups - home based:
al-Nusrah Front: aim(s): overthrow Syrian President Bashar al-ASAD's regime, absorb like-minded Syrian rebel groups, and ultimately, establish a regional Islamic caliphate
area(s) of operation: headquartered in the northwestern Idlib Governorate, with a minor presence in Halab Governorate; operational primarily in northern, western, and southern Syria; installs Sharia in areas under its control; targets primarily Syrian regime and pro-regime forces, some minorities, other Syrian insurgent groups, and occasionally Western interests (2018)
Hay'at Tahrir al-Sham (HTS): aim(s): an alias of the al-Nusrah Front; overthrow Syrian President Bashar al-ASAD's regime, absorb like-minded Syrian rebel groups, and, ultimately, establish a regional Islamic caliphate
area(s) of operation: Northwest Syria (2018)
Islamic State of Iraq and ash-Sham (ISIS): aim(s): replace the world order with a global Islamic state based in Iraq and Syria; expand its branches and networks in other countries; rule according to ISIS's strict interpretation of Islamic law
area(s) of operation: ISIS has lost most of the territory it once controlled and now its overt territorial control is limited to pockets of land along the Syria-Iraq border and in southern Syria (2018)

Terrorist groups - foreign based:
Abdallah Azzam Brigades (AAB): aim(s): disrupt and attack Shia Muslim and Western interests in Syria
area(s) of operation: remains operational; conducts attacks against primarily Shia Muslim organizations and individuals, including Hizballah members, and Westerners and their interests (2018)
al-Qa'ida (AQ): aim(s): overthrow President Bashar al-ASAD's regime; establish a regional Islamic caliphate and conduct attacks outside of Syria
area(s) of operation: operational primarily in Idlib Governorate and southern Syria, where it has established networks and operates paramilitary training camps (2018)
Ansar al-Islam (AAI): aim(s): remove Syrian President Bashar al-ASAD from power and establish a government operating according to sharia
area(s) of operation: operationally active in Syria since 2011; launches attacks on Syrian Government security forces and pro-Syrian Government militias; some AAI factions combat ISIS, while others are aligned with ISIS (2018)
Hizballah: aim(s): preserve Syrian President Bashar al-ASAD's regime
area(s) of operation: operational activity throughout the country since 2012; centered on providing paramilitary support to President Bashar al-ASAD's regime against armed insurgents (2018)
Islamic Revolutionary Guard Corps -- Qods Force (IRGC-QF):

aim(s): assist government forces in suppressing opposition forces and Islamic State of Iraq and ash-Sham (ISIS) forces; train Syrian Government troops; conduct strikes against Israel; funnel arms and money onward to Lebanese Hizballah
area(s) of operations: throughout Syria (2019)
Kata'ib Hizballah (KH): aim(s): preserve Syrian President Bashar al-ASAD's regime
area(s) of operation: deploys combatants to Syria to fight alongside Syrian Government and Lebanese Hizballah forces (2018)
Kurdistan Workers' Party (PKK): aim(s): advance Kurdish autonomy, political, and cultural rights in Syria, Turkey, Iraq, and Iran
area(s) of operation: operational in the north and east; majority of members inside Syria are Syrian Kurds, along with Kurds from Iran, Iraq, and Turkey (2018)
Mujahidin Shura Council in the Environs of Jerusalem (MSC): aim(s): destroy the state of Israel; enhance its networks in Syria
area(s) of operation: maintains limited networks for operational planning against Israel (2018)
Palestine Liberation Front (PLF): aim(s): enhances its networks and, ultimately, destroy the state of Israel and establish a secular, Marxist Palestinian state with Jerusalem as its capital
area(s) of operation: maintains a recruitment and training presence in many refugee camps (2018)
PFLP-General Command (PFLP-GC): aim(s): preserve Syrian President Bashar al-ASAD's regime
area(s) of operation: maintains a political base in Damascus; fights with President al-ASAD's forces and Hizballah in areas where anti-regime paramilitary groups are active (2018)
Popular Front for the Liberation of Palestine (PFLP): aim(s): enhance its recruitment networks in Syria
area(s) of operation: maintains a recruitment and limited training presence in several refugee camps (2018)