Back in July, an idea by Kalle Lasn and his colleagues at Adbusters, a nonprofit magazine run by social activists, had started to come together. For months, Lasn had noticed among his 120,000 readers an unresolved anger that wasn’t finding expression. He observed that young people were starting to say they worried about having a “black hole future” ahead of them, and it suddenly felt, he said, “like a Tahrir moment in America was eminently possible.” So the Adbusters team tried something out. They put out feelers for a small protest on Wall Street on Sept. 17. They started a...