WASHINGTON, March 15 - The Department of Homeland Security, trying to focus antiterrorism spending better nationwide, has identified a dozen possible strikes it views as most plausible or devastating, including detonation of a nuclear device in a major city, release of sarin nerve agent in office buildings and a truck bombing of a sports arena. The document, known simply as the National Planning Scenarios, reads more like a doomsday plan, offering estimates of the probable deaths and economic damage caused by each type of attack. They include blowing up a chlorine tank, killing 17,500 people and injuring more than 100,000;...