Tehran has long wielded a strong influence over the war-torn country and exploited it for its foreign policy gains.
The use of chemical weapons is something that riles the collective conscience of Iranians.
Iran is the world’s biggest laboratory of surviving chemical attack victims. More than 50,000 people, many of them former soldiers, still suffer from the paralysing effects of nerve agents sarin and tabun used by Saddam Hussain’s forces during the Iran-Iraq war in the 1980s.
It’s customary for foreign journalists visiting Tehran to be taken to the Peace Museum and see the pictures of the devastation caused by the use of chemical weapons.
But in Syria, the Iranian government looked the other way and continued to support the regime of Bashar al Assad, which used chlorine and sarin against civilians more than 100 times.
Iran’s military, economic and diplomatic backing has been crucial for Assad’s survival in the eight-year-long civil war that has killed half a million people and displaced millions more.
Despite international sanctions from the United States and the European Union, Tehran continued to train and arm Assad’s army, which carried out a brutal crackdown against the opposition after the 2011 pro-democracy demonstrations.
Iran-backed Shia militias fought alongside regime forces in towns and cities dominated by Sunni Arabs. Assad belongs to the Alawite sect, an offshoot of Shia Islam, that has controlled the government and bureaucracy for decades.
Iranian militias have faced accusations of attacking civilians such as during the capture of Aleppo.
But Iran’s involvement in Syria goes beyond sectarian considerations, as for Tehran Damascus acts as a crucial route to build regional influence across the Middle East.
Iran’s involvement is also driven by business interests. With the post-war reconstruction of Syria starting to gain momentum, thanks to the Astana Peace Process, Tehran seeks to build big infrastructure projects, even though it would mean competing with Russia.
Israel is unnerved by Iran's regional powerplay and has already hit Iranian targets inside Syria a few dozen times, leaving the country vulnerable to another phase of regional conflict.
For Iran, Syria also acts as a conduit to supply weapons to Lebanon-based Hezbollah.
Since defeating Daesh, Iran has deepened its influence by offering money, food, and ID cards to Syrians who agree to convert to Shia Islam, according to a report in The Wall Street Journal.
This is problematic for Syria’s Arab neighbours who view Iran as a foe. The attacks this year on oil tankers off the coast of the United Arab Emirates in the Gulf have further heightened the tensions.
Iran has spent billions of dollars and sent thousands of fighters to bolster Assad’s grip on Syria. And so it was no surprise when it opposed Turkey’s decision to launch an operation in northern Syrian against the YPG terrorist group.
From the Turkish point of view, Syria has not been able to contain a group that poses a threat to its neighbours and Ankara has decided to take on the challenge itself.
“Iran asking Turkey to respect Syria's territorial integrity is instrumental to Iran's interests,” says Dr Mauro Gilli, a senior researcher at the Center for Security Studies in Zurich.