Complex sentence: Operation Mockingbird was a campaign by the United States Central Intelligence Agency (CIA) to influence media known for a fact to have operated at least during the 1950s, 1960s, and 1970s.
Split into simpler sentences: Operation Mockingbird was a campaign by the United States Central Intelligence Agency (CIA) to influence media. The operation has been documented as operating at least during the 1950s, 1960s, and 1970s.