The U.S. war in Iraq enters its fifth year today. That, and 3,197 U.S. military deaths reported by the Pentagon as of 10 a.m. Friday, are among the few numerical certainties in a conflict characterized from the start by confusion and misuse of key data.

In the fog of modern counterinsurgency warfare, statistics have replaced conquered territory as measures of success. Then-Defense Secretary Donald H. Rumsfeld once dismissed questions about the level of combat-ready Iraqi troops by saying that numbers are only numbers and "misleading" as to the truth, but the Bush administration has supplied a steady stream of them...