AT THE MOSUL DAM, Iraq -- The largest dam in Iraq is in serious danger of an imminent collapse that could unleash a trillion-gallon wave of water, possibly killing thousands of people and flooding two of the largest cities in the country, according to new assessments by the U.S. Army Corps of Engineers and other U.S. officials. Even in a country gripped by daily bloodshed, the possibility of a catastrophic failure of the Mosul Dam has alarmed American officials, who have concluded that it could lead to as many as 500,000 civilian deaths by drowning Mosul under 65 feet of...