Syria is a proxy war.
Experts on the Levant often brandish this term in an attempt to justify why the ongoing civil conflict in Syria continues to roil in bloodshed, causing more than 110,000 deaths in its two and a half years.
Foreign intervention is one of the chief causes behind its longevity, they say. American support for logistics, humanitarian aid and, recently, weapons has certainly bolstered the ability of rebel opponents to take on the military strength of the Bashar Assad regime.
But many with intimate knowledge of the region nod toward the more subversive intentions of other regional powers. Iran and Saudi Arabia have a series of supply routes into that country, as well as other methods of support.
Here's what you need to know about those two country's involvement in the ongoing war in Syria:
Iran is predominantly Shiite Muslim. Saudi Arabia is predominantly Sunni.
"Their differences go back almost as far as the founding of Islam," says Theodore Kattouf, a career diplomat who served as U.S. ambassador to Syria in the early 2000s. "Saudi leaders have a conservative Sunni identity, and Iranian leaders have a Shiite identity."
Sunni Islam was the first major sect of the religion and represents the belief system for a vast majority of Muslims worldwide. The original division with the Shiite Muslims originates with a debate over who directly succeeded Mohammad, the prophet of God, as the second Imam or spiritual leader of the faithful.
"Both sides are capable of cynically employing these identities to motivate their partisans to fight one another," says Kattouf.
Divisions between the two sects have magnified culturally since Islam began in the 7th Century.
Roughly three-quarters of Syria is comprised of Sunni Muslims. It has been ruled, however, since 1971 by the Assad family who are ethnic Alawites, a Shiite sect that has been historically persecuted by the Sunnis.
"Saudi Arabia sees Syria as an overwhelming Sunni Muslim country in which Shiite Iran has propped up a minority-dominated regime," says Frederic Hof, an ambassador and political appointee who served as special adviser to President Barack Obama for the transition in Syria.
"When the regime reacted violently to the initial waves of peaceful protests in 2011, Saudi Arabia processed those events as Iran-abetted sectarian slaughter directed at Arab Sunnis," he says.
Saudi Arabia sees Syria as a vital member of the Arab community, says Hof, now a senior fellow at the Atlantic Council, and believes Iran's intervention in Syria is a "totally inadmissible symptom of Iran's attempt to penetrate the Arab world."
Saudi Arabia is also home to Mecca and Medina, two of the most sacred places for Muslims. Iran hopes to rob it of some of that influence, says Kattouf, now president of Amideast.org.
"[Iran] has become the defacto Shiite power in the region," he says. It hopes to be "in the vanguard of all Islamic groups" in the region, by challenging Saudi Arabia for its holy places, and trying to appeal to Sunnis as well as Shiites.
Syria at one point was considered the center of the known world, not in the least for its geographic position at the crossroads of ancient cultures.
(And, speaking of religion, Damascus was one of the first places to receive the Christian ministry of St. Peter).
It remains a key strategic point, particularly for Iran.
Reza Kahlili is the nom-de-plume for a native Iranian, who claims to have served in the elite Iranian Revolutionary Guard Corps before defecting to spy for U.S. intelligence services.
"[Syria] is the corridor that connects Lebanon and Iraq, and the combination of activities in both regions, with the hopes of creating instability in Jordan and crushing Israel," he tells U.S. News.
Kahlili says the Guard has been operating in Syria through small bases since the early daysof Iran's revolutionary government. It has established command and control centers and monitors Islamic extremist movements from there. It also trains and supplies organizations such as Hezbollah, the Lebanese militant political party that has sent an undisclosed number of fighters in support of the Syrian regime.