Iraq's Shi'ite Prime Minister Nouri al-Maliki has long warned that Syria's increasingly sectarian war might spill over the border and reignite his own country's combustible Shi'ite-Sunni mix.
That nightmare may have edged closer after suspected Sunni insurgents killed 48 Syrian troops on Iraqi soil on Monday.
Suicide bombers have already stepped up attacks in recent weeks to a frequency Iraq has not suffered in years.
Invigorated by the conflict in neigboring Syria, insurgents are gaining ground and recruits in Iraq's Sunni heartland, regrouping in the vast desert where the Euphrates river winds through both countries, security officials say.
"We warn all sides in Syria against moving their armed struggle onto Iraqi lands or violating the sanctity of its borders," Iraq's defense ministry said after the attack on the Syrians, which it blamed on infiltrators from Syria. "The response will be harsh and decisive."
Syria's crisis has always been delicate for Iraq's Shi'ite leadership. Baghdad is close to Iran, Syrian President Bashar al-Assad's ally, but insists it takes no sides as the conflict next door widens a regional Shi'ite-Sunni divide.
Iraq says the Syrian soldiers had fled into its territory and were being escorted back when they were ambushed. Yet their incursion will raise questions about Baghdad's neutrality.
Insurgents in Syria are predominantly Sunni and are backed by Sunni regional powers such as Turkey, Saudi Arabia and Qatar, while Assad's minority Alawite sect springs from Shi'ite Islam.
For Maliki, a defeat for Assad threatens to put Damascus under the thumb of hardline Sunni Islamists hostile to Baghdad.
Piggybacking on Syria Conflict
Iraq's recent surge in violence is still well below the peak of sectarian slaughter in 2006-2007, when rival Sunni and Shi'ite Islamist militias ruled parts of Baghdad, tens of thousands were killed and suicide bombers struck daily.
However, insurgents in suicide vests and explosive-packed cars have struck almost twice a week since January, killing more than 230 people in Shi'ite districts and mixed areas disputed by Iraqi Arabs and ethnic Kurds.
Security officials believe insurgents are tapping into Sunni discontent in Iraq's western provinces, where for weeks thousands have protested against Maliki, accusing him of marginalising their sect since the fall of Saddam Hussein.
Reviling the Shi'ite-led government it sees as oppressing Sunnis, the al-Qaida-linked Islamic State of Iraq group is trying to gain legitimacy by linking its struggle to the Sunni insurgency against Assad, security experts say.
Protesters in the Sunni stronghold of Anbar have raised flags of the Syrian opposition.
"We say to the Sunnis in Baghdad and elsewhere: the situation you are living today is exactly what the mujahideen warned you of years ago. You are walking in a dark tunnel," said an al-Qaida statement posted on an Islamist website.
Iraqi security officials believe Islamic State of Iraq and other Sunni Islamist insurgents have already started to make good on a vow to reclaim ground al-Qaida lost in western Iraq.
"There is something even more immediate - the opportunity to merge Syria and Iraq into a single sectarian theatre of conflict," said Ramzy Mardini, an author on Iraq's insurgents who is now at the Iraq Institute for Strategic Studies. "Sunni extremists in Iraq see an opportunity to piggyback off the Syrian revolution."
New al-Qaida Camps
In Anbar province, which forms a third of Iraqi territory and was once almost wholly held by al-Qaida, tribal ties transcend the border. Sunni chiefs say Iraqi tribes send Syrian relatives food and supplies. Some tribal leaders say they send arms to Free Syrian Army rebels when border controls allow.
U.S. officials say the Nusra Front, an Islamist group seen as one of the most active fighting forces in Syria, is closely linked to al-Qaida's Iraqi affiliate.
Iraqi security officials say controlling armed groups in the region has been hampered by Sunni protesters who accuse security forces of unfairly targeting them with anti-terrorism laws. To avoid possible clashes, the army has pulled back in some areas.
Maliki has warned Sunnis against allowing extremists to hijack protests and has sought to appease them by promising to modify laws and by releasing detainees. But even moderate Sunni leaders and sheikhs worry they are losing influence.
Iraqi security officials and local tribal leaders say new al-Qaida camps are emerging in the remote al-Jaziya desert and valleys along the Syrian border in Anbar, but also that small cells are returning to towns such as Falluja and Rutba.
"In the last operation, we targeted a camp and managed to kill more than 10 members and to seize stocks of explosives and weapons. What stood out was the new weapons and what seems to be continuous support," said one army intelligence officer.
One local Sunni sheikh with close contacts to insurgents said al-Qaida cells were once again using militant rhetoric to attract young men for suicide attacks to exact revenge for wronged relatives and perceived abuses against their sect.
Security officials acknowledge they lack local intelligence in places such as Anbar, where state forces are seen by many as tools of the Shi'ite-led government. They also miss U.S. air surveillance over the desert bordering Syria.
Shi'ite Militias on Board
So far Shi'ite militias have stayed mostly out of the fray.
But in Sadr City, the Baghdad bastion of Shi'ite militants who once battled U.S. troops, former fighters talk of remobilizing in case Syria's turmoil upsets Iraq's sectarian balance.
After the last U.S. troops left Iraq in December 2011, some Iranian-backed, anti-U.S. Shi'ite militias disbanded, at least officially saying their struggle was over after nearly a decade.
With the rising influence of their political parties, there seemed little reason for the Shi'ite militants to keep fighting.
Early last year the leader of one militia, Asaib al-Haq, said it would disarm and join the political process. A rival Shi'ite group, Kataeb Hizballah, said weeks later it would not follow suit, citing Iraq's continuing instability.
The Mehdi Army of anti-U.S. Shi'ite cleric Muqtada al-Sadr mostly disbanded after its defeat by Iraqi and U.S. forces in Baghdad and southern cities in 2008.
But Syria's crisis has revived Iraqi Shi'ite militant activity. Some have crossed the border to fight alongside Assad's troops, though Shi'ite militia commanders say they have given no official sanction for volunteers to fight in Syria.
Worried about Sunni unrest in Anbar and the crisis unfolding in Syria, some ex-Mehdi Army members say they too are regrouping and recruiting as a precautionary measure.
"I will just be defending my rights," said one former senior Mehdi fighter. "We won't start this. We will just be waiting for them in case."