WikiLeaks founder Julian Assange has brought the U.S. "to its knees", former Cuban leader Fidel Castro said in an article published on Wednesday. "Assange - a man who was known by few just several months ago - has proved that a challenge could be issued to even the most powerful empire ever," Castro's Empire in the Dock article read. "Ideas can be more powerful than nuclear weapons," he added. World leaders and diplomats have said some of the more than 250,000 confidential U.S. diplomatic cables made public by WikiLeaks could "threaten lives." Assange was arrested in London on December 7 and reportedly released on bail on Wednesday. His extradition to Sweden, where he is wanted on sex assault charges, is pending. Assange denies the allegations and is fighting against extradition, while his lawyers fear that he could be then extradited to the United States to face the death penalty for violation of the Espionage Act of 1917.