After September 11, 2001, as part of a general increase in security, dozens of U.S. government web sites were closed to the public (or just shut down.) These sites provided information to the public about things like airports, power plants, military bases and infrastructure in general. However, a closer examination of 5,000 federal government web pages, containing information on potential targets, found that only four of these pages had data that could not be found somewhere else and might be of use to terrorists. Since 2002, a lot of the web sites that were taken down, were put back on the net. It was found that many Americans had legitimate need for the information.
The tendency to classify information on the basis of suspected vulnerability is an old one. In the 1960s, the Department of Defense classified massive amounts of information on the mistaken assumption that it was harmful if the enemy (the Soviet Union and communist China) knew that we knew what their army uniforms looked like, or what the range of their tank guns was. Unfortunately, the policy also kept American troops in the dark about basic information on potential battlefield opponents. Combat officers knew this policy was idiotic and counterproductive, and it took a decade of complaining to get it changed.
This time around, it only took a few years.