Shortly after midnight Wednesday, a homemade bomb exploded here in the capital, a harbinger of mayhem. Around 5 a.m., a truck bomb exploded in Kadhimiya, a Baghdad neighborhood where Shiite pilgrims had begun to gather to commemorate the life and death of a revered imam who was the Prophet Muhammad’s great-grandson. Then, reports of other attacks flooded in from around the country — Samarra, Kirkuk, Mosul, Falluja, Ramadi, Hilla — and by midday officials said more than 90 people were dead and at least 260 were wounded.