A predawn bomb attack wrecked an important Shia shrine in Samarra on Wednesday, sparking widespread protests and forcing the government to issue an urgent appeal to avoid sectarian reprisals.
Tens of thousands of people staged protests across Iraq after the detonation of two bombs inside the holy shrine. Dozens of Sunni mosques were later reported to have been targeted with six Sunnis killed.
The most dread of terms - civil war - is beginning to be whispered by Washington policymakers and pundits. If the violnce escalates into a factional, countrywide war, hopes that American troop withdrawals would begin this year will be dashed, and it is likely that troop strength in Iraq would need to be increased to quell the violence.
If the flames of ethnic and religious violence could be quenched at all.
In the southern city of Basra Shia militiamen attacked a prison and lynched at least eleven Sunni inmates. Among the victims were at least two suspected foreign insurgents.
The shining dome of the 1200-year old Askariya shrine crumbled to the ground, and leaders on both sides called for peace.
"We are facing a major conspiracy that is targeting Iraq's unity," said President Jalal Talabani, a Kurd. "We should all stand hand in hand to prevent the danger of a civil war."
Shiite Grand Ayatollah Ali al-Sistani issued a statement forbidding attacks on Sunni mosques and calling for seven days of national mourning. He broke with past practice, though, and also called for public protests.
Respected Middle East scholar Juan Cole was especially disheartened by the events:
Tuesday was an apocalyptic day in Iraq. I am not normally exactly sanguine about the situation there. But the atmospherics are very, very bad, in a way that most Western observers will miss.
Strap yourselves in, folks. I believe that the situation in Iraq is about to take a sharp nosedive into chaos.