There are reasons to be wary of wind turbines—they're dangerous to transport, emit unpleasant noises, and have the nasty habit of killing birds and bats that come crashing into them. But we never thought that turbine projects could be considered a threat to the military.
The New York Times reports that Dr. Dorothy Robyn, deputy under secretary of defense, has declared that wind turbines pose an unacceptable risk to national security and military training in some areas—because moving turbine blades can cause "blackout zones" where planes disappear from radar. And in certain cases, turbine blades are indistinguishable from approaching planes.
The problem apparently caused 9,000 megawatts worth of wind turbine plans to be scrapped in 2009, and the issue isn't easing up. It's a purely technological issue—potential fixes include new coatings for wind turbines, software patches for existing radar systems, and completely overhauling radar systems (many are over 50 years old).
In the meantime, we have a suggestion for the government: if it's so easy to trick aircraft radar systems, it might be worth leveraging the loophole to spy on wind turbine-enshrouded enemy territory.