Why it cares:
Iran and Syria are bound by two factors: religion and strategy.
a) Religion: Iran is the world's most populous Shiite Muslim nation. The Syrian government is dominated by Alawites, a Shiite offshoot, and the rebels are dominated by Sunnis.
That connection has bound them for quite a while. Iran counted on Syria as its only Arab ally during its eight-year war with Iraq. Iraq was Sunni-dominated.
The last thing Iran wants now is a Sunni-dominated Syria -- especially as the rebels' main supporters are Iran's Persian Gulf rivals: Qatar and Saudi Arabia.
b) Strategy: For Iran, Syria is also a strategically key ally. It's Iran's main conduit to the Shiite militia Hezbollah in Lebanon, the proxy through which Iran can threaten Israel with an arsenal of short-range missiles.
In 2009, the top U.S. diplomat in Damascus disclosed that Syria had begun delivery of ballistic missiles to Hezbollah, according to official cables leaked to and published by WikiLeaks.
So, it's in Iran's interest to see al-Assad's regime remain intact.
Western intelligence officials believe the Islamic Republic has provided technical help such as intelligence, communications and advice on crowd control and weapons as protests in Syria morphed into resistance.
A U.N. panel reported in May that Iranian weapons destined for Syria but seized in Turkey included assault rifles, explosives, detonators, machine guns and mortar shells.
Ayham Kamel of Eurasia Group believes the Iranians must be alarmed that the tide is turning against al-Assad.
"Iran probably has excellent information regarding Assad's position. That information would make clear that Iran is increasingly likely to lose its only ally in the region, greatly reducing its strategic reach," he said.
What's it saying:
Iran has cast events in Syria as part of a much broader ideological battle. It's a "war between the front of hegemony and the front of resistance," Supreme Leader Ayatollah Ali Khamenei has said.
Iran's position, as outlined by Foreign Minister Javad Zarif and new President Hassan Rouhani, is that the Syrian government is a victim of international plots.
Iran believes the West and almost all Arab countries are in cahoots in an effort to implement regime change in Syria. Iran says the main objective of this plot is to make the region safer for Israel.
This week, Zarif warned of "graver conditions" in Syria is attacked.
"If any country attacks another when it wants, that is like the Middle Ages," Zarif said Wednesday.
Why it matters:
Many believe Iran is Washington's greatest threat in the region, especially with its nuclear potential. It's unclear how Iran might respond if Syria is attacked. But the rhetoric certainly has been ominous.
"Starting this fire will be like a spark in a large store of gunpowder, with unclear and unspecified outcomes and consequences," Khamenei told Iranian Cabinet members this week.
"The U.S. threats and possible intervention in Syria is a disaster for the region and if such an act is done, certainly, the Americans will sustain damage like when they interfered in Iraq and Afghanistan."
Why it cares:
China's relationship with Syria is more nuanced.