BAGHDAD — Ripping open an envelope containing a small, hard object, Sadoun Dulame discovered the unwanted gift Iraq's academics have learned to dread. "They sent me a bullet," he said, describing the letter he received last month. "They said in Arabic: 'You cost us just one bullet, no more, so shut your mouth'." Death threats and assassinations are teaching Iraqi academics to watch what they say. Iraq's new interim government says 31 university lecturers have been murdered since last year's US-led invasion and many more have received warnings to keep quiet. While the motives for the killings vary, academics argue...