AT THE START of the 21st century, everyone agreed that the next big thing was clean technology. It had to be: in Beijing, the smog had gotten so bad that people couldn’t see from building to building—even breathing was a health risk. Bangladesh, with its arsenic-laden water wells, was suffering what the New York Times called “the biggest mass poisoning in history.” In the U.S., Hurricanes Ivan and Katrina were said to be harbingers of the coming devastation from global warming. Al Gore implored us to attack these problems “with the urgency and resolve that has previously been seen only when nations mobilized for war.” People got busy: entrepreneurs started thousands of cleantech companies, and investors poured more than $50 billion into them. So began the quest to cleanse the world. It didn’t work. Instead of a healthier planet, we got a massive cleantech bubble. Solyndra is the most famous green ghost, but most cleantech companies met similarly disastrous ends—more than 40 solar manufacturers went out of business or filed for bankruptcy in 2012 alone. The leading index of alternative energy companies shows the bubble’s dramatic deflation: Why did cleantech fail? Conservatives think they already know the answer: as soon as green energy became a priority for the government, it was poisoned.