President Barack Obama is a foreign-born radical plotting to establish a dictatorship. His predecessor, George W. Bush, allowed the Sept. 11 attacks to occur in order to justify sending U.S. troops to Iraq. The federal government has plans to imprison political dissenters in detention camps in the United States. Welcome to the world of conspiracy theories. Since colonial times, conspiracies both far-fetched and plausible have been used to explain trends and events ranging from slavery to why U.S. forces were surprised at Pearl Harbor. In today's world, the communications revolution allows conspiracy theories to be spread more widely and quickly than ever before. But facts that undermine conspiracy theories move less rapidly through the Web, some experts worry. As a result, there may be growing acceptance of the notion that hidden forces control events, leading to eroding confidence in democracy, with repercussions that could lead Americans to large-scale withdrawal from civic life, or even to violence.