The six-year civil war in Syria has propelled relations with Iran to a new level of importance on a par, according to some Iranian officials, with Iran’s control over parts of its own territory.

In Iran’s security doctrine, Syria serves as strategic depth to deflect attacks from Israel, the Islamic State or a Saudi-led Sunni Muslim alliance.

Iran-Syria relations have been close since 1980 when Syria was the only Arab country to back Iran in the Iran-Iraq war. But the protests that broke out in Syria against the regime of Bashar al-Assad six years ago have created a new level of intimacy and dependence.

On Feb. 14, 2013, Hojatoleslam Mehdi Taeb, the head of the Ammar Strategic Base — an organization established to fight the “soft war” against the Islamic Republic and an adviser to the Supreme Leader—went so far as to declare that Syria is the 35th province of Iran. Taeb said Syria’s strategic importance exceeds that of Khuzestan, a largely ethnic Arab province in southern Iran.

While other Iranian officials do not go this far in public, the notion that Iran needs strategic depth has long been popular with the Revolutionary Guards and its overseas arm, the Quds (Jerusalem) Force. The late leader of the Islamic Revolution, Ayatollah Ruhollah Khomeini, advocated exporting the revolution to neighboring Sunni-ruled countries and beyond, a task enshrined in Articles 152 to 155 of the Constitution. Not codified but equally weighty was Khomeini’s command to liberate Jerusalem and Palestine from the “Zionist enemy.”

Hassan Abbasi, the head of the aptly named Center for Borderless Doctrinal Analysis, the think tank of the Revolutionary Guards, wrote about this strategy six years ago.  He argued that the Islamic Republic could not be safe unless it persuaded other Muslim nations to take the same path. “If we remain alone, we will always be in danger,” he said. “Our system will also be in danger if most Muslim nations take the path of Western-style democracy.”

According to the Guards’ doctrine, Iran’s mission allows it to override borders and state sovereignty, which Abbasi described as a colonial invention.

In tactical terms, the regime has relied on an extensive network of groups to extend its influence. The crown jewel is Hezbollah. Starting in the early 1980s, the Guards and the Quds Force built a small Shi’ite militia in Lebanon into a formidable fighting force that has survived wars with Israel. In addition, Hezbollah has offered a cadre of seasoned fighters who assist the Guards and Quds Force in combat, training and terrorist attacks. Hezbollah and the Gaza Strip-based Hamas and Palestinian Islamic Jihad have served as the vanguard force fighting Israel. Equipped with an array of short and medium range missiles capable of reaching Tel Aviv, Hezbollah is considered a deterrent to a possible Israeli attack on Iran.

The Assad regime has long offered a land and air bridge for transporting Iranian arms and munitions to Hezbollah. The civil war could have caused the demise of the Alawite regime and severed these links. To prevent this outcome, Iran forcefully intervened.

Although the government of President Hassan Rouhani has notably been less enthusiastic about the war, Supreme Leader Ayatollah Ali Khamenei sided with the Guards. Recent hostile rhetoric from the Trump administration and the Saudis, as well as terrorist attacks in Tehran in June, appear to have solidified regime support for the Syria intervention.

By any measure, that intervention has been Iran’s largest to date. Some 8,000 to 10,000 fighters from the Revolutionary Guards have participated. Iran has lost more than 1000 fighters, including 18 high-ranking generals. Hezbollah’s losses are estimated at 1,045 and a similar number of Afghan, Pakistani and other Shi’ite militiamen have also died. The financial cost to Iran is estimated at $15 to 20 billion  .

In a sign of the significance of the Syrian campaign, the Guards deployed the Saberin Brigade, a special force modeled on the elite British Special Air Force. With the fall of Aleppo and the shrinking of other rebel-held territories, the investment seems to have paid off. So much so that the regime has been turning Syria into a neo-protectorate.

Recognized in international law, a classic protectorate arrangement entails a stronger state protecting a weak one, in return for certain political, economic or strategic privileges. A neo-protectorate would preserve Syria’s sovereignty, but its territory, borders, foreign policy and security apparatus would come under considerable control of Iran.

Some elements of this neo-protectorate are already in place. Iran has acquired extensive rights to naval facilities in Latakia for its Mediterranean based fleet. The Quds Force, under its energetic commander Qassem Suleimani, is developing plans to use Syrian bases for training a newly announced Shi’ite Liberation Army made up of recruits primarily from Afghanistan, Pakistan and Iraq as well as Syrian Alawites. While it is not clear how large this force will be, its existence is seen as a potential threat to Saudi Arabia, Bahrain and other Arab countries with large Shi’ite populations. It also contradicts Iran’s insistence that it does not intervene in neighboring countries unless requested by their governments.

Security cooperation between Iranian and Syrian intelligence and security forces has long been extraordinarily tight. The Quds Force’s Unit 400, a Special Operations force commanded by Maj. Gen. Majid Alavi, a former deputy Iranian intelligence minister, Maj. Gen. Hamed Abdollahi and Maj. Gen. Mohsen Chizari, has collaborated with the feared Syrian Mukhabarat. The Mukhabarat, which has penetrated virtually all the groups fighting in Syria, has provided intelligence to the Iranians and may have helped in some false flag operations in the region. One possible case, according to the Turkish National Intelligence Organization, is that of Uzbek national Abdulgadir Masharipov. The Istanbul nightclub attacker, who killed 39 people on New Year’s Day 2017, entered Turkey via Iran.

This is not to say that Iran’s project has not faced some pushback. Israel, which has repeatedly bombed Iranian arms shipments in Syria meant for Hezbollah, has declared that it will not tolerate a “Syrian Hezbollah” on its border. Russia, which has close ties with Israel as well as Syria, reached an agreement with the Trump administration for a cease-fire in southwestern Syria, which went into effect on July 9. This is part of a broader agreement to create “de-confliction zones” that keep Iran and Hezbollah away from the Golan Heights and the Jordanian border. But with the US reluctant to intervene more deeply in Syria, Iranian influence is there to stay.

Farhad Rezaei is a research fellow at Center for Iranian Studies (IRAM) in Ankara where he researches Iran’s foreign policy. He is the author of Iran’s Nuclear Program: A Study in Proliferation and Rollback (New York: Palgrave Macmillan, 2016). His writings have appeared in Harvard-Iran Matters, Atlantic Council, and the National Interest among others. His new book is Iran’s Foreign Policy after the Nuclear Agreement: The Politics of Normalizers and Traditionalists, Palgrave Macmillan, forthcoming. He tweets at @Farhadrezaeii