A Muslim solder, Nidal Hasan, shot dead 13 people at Fort Hood yesterday. Hasan had earlier exhibited extremist, anti-American propensities, including applauding terrorist attacks against U.S. soldiers. There are different theories as to how this could have happened.
One school of thought attributes the tragedy to politically-correct double standards imposed on the military that kept the alarm bells from going off.
These explanations are not mutually exclusive. Doubtless other factors could have contributed to the tragedy as well.