Three time zones, 3,000 miles, and a cultural galaxy apart, New York and Los Angeles face a common threat: along with Washington, D.C., they’re the chief American targets of Islamic terror. And both cities boast top cops, sometime rivals—the cities are fiercely competitive—who know that ensuring that a dog doesn’t bark will determine their legacies. After investing millions of dollars in homeland security, Police Commissioner Raymond W. Kelly of New York and Chief William J. Bratton of L.A. can both claim counterterror successes. What can we learn from their approaches? And will they be able to continue preventing terrorist attacks in their cities?