Iraqi Shiites, like their allies in Iran, fret that unrest in Syria could oust President Bashar Al-Assad and bring to power hard-line Sunnis eager to put their weight behind fellow-Sunnis in Iraq who have lost out since Saddam Hussein's fall.
"If Syria falls, Iraq will work with Iran to influence events in Syria," said a senior Iraqi Shiite politician, who asked not to be named.
"Change in Syria will cause major problems for Iraq. They (Sunnis) will incite the western (Sunni) part of Iraq."
They fear the turmoil next door could spill into Iraq, reignite sectarian violence and intensify a proxy battle between Iran and Saudi Arabia, which sees itself as the bastion of Sunni Islam and has never come to terms with Shiite rule in Baghdad.
Iraqi Shiite militias are unlikely to fight for Assad's survival, but might respond if Sunnis in Iraq's western Anbar province are emboldened by the rise of Sunni power in Syria.
Syria was the only Arab nation to side with Iran in the 1980-88 war with Iraq. Assad's minority Alawite sect is a distant offshoot of Shiite Islam. Syria links Iran logistically with its Lebanese Shiite Hezbollah guerrilla protégés.
Despite violent repression that, by a UN count, has killed 2,700 people, Assad has failed to quell six months of protests mostly involving majority Sunnis. The anti-Assad movement has sought to shun any sectarian agenda, but Sunni Islamists may emerge as a significant force if the president is deposed.
"Will Iran, Hezbollah in Lebanon and their allies in Iraq stand idle and watch Assad collapse and thus bring down with him one of the pillars of the 'Shiite Crescent' without reacting? Impossible," said Iraqi political analyst Ibrahim Al-Sumaidaie.
"Saudi Arabia and the Sunni Arab powers will try to redraw the political map in Iraq to increase Sunni influence in decision-making because until now they have not made peace with Shiite rule here," he said.
To protect their interests, Syria, Hezbollah and Iran would "try to unsettle the security situation in Iraq through their relationships with Shiites and Sunnis", Sumaidaie added.
Since the fall of Saddam, a bitter foe of Tehran, some Sunni rulers in the Middle East have talked of the emergence of a ‘Shiite Crescent’ running from Iran through Iraq and Alawite-ruled Syria to Hezbollah-controlled Lebanon.
Iran, projecting a pan-Islamic image, rejects any such sectarian concept, referring instead to an ‘axis of resistance’.
The Islamic Republic's influence, which has burgeoned since the US-led war in Iraq, would take a damaging hit if Assad fell, with repercussions for Iraq and the rest of the region.
Iraq is trying to end a legacy of sectarian violence that drove it to the brink of civil war in 2006-07; just as US troops prepare to complete their withdrawal by 31 December.
It has taken a muted stance on Syria, while Saudi Arabia and some other Arab nations have condemned Assad's crackdown.
Iraqi Shiite politicians can give Assad diplomatic and financial support, said the senior politician, "Because we don't want a government inimical to Iraq to be installed in Damascus."
He said Iraq could provide intelligence on people and arms being smuggled across the border, and ensure ample trade and financial ties with Syria, which last week banned most imports in an effort to conserve dwindling foreign currency reserves.
Syria, which also borders Israel, Lebanon, Turkey and Jordan, has some influence in Iraq, although the two countries feuded when each was ruled by rival wings of the Baath Party.
Saddam's fall in 2003 brought majority Shiites to power in Iraq, which now has better ties with Iran and Syria, allies in a regional power struggle against US-backed Sunni-ruled states.
Last year Iran brokered a deal between Iraq's main Shiite factions, helping them tighten their grip on power, with the blessing of Syria and Turkey, guaranteeing another term for Prime Minister Nuri Al-Maliki, a Shiite, Iraqi politicians say.