By historical importance, I mean outside of importance in terms of the film industry or in cultural trends. Birth of a Nation is said to have been a primary reason that the second Ku Klux Klan developed. And while not a film, I remember reading that a television event series based off of the Mahabharata in India in the 1988 ended up leading to a Hindu nationalist resurgence that culminates with the destruction of the mosque in Ayodha in 1992 (said to have been a temple marking the birthplace of Rama destroyed by the Mughals), leading to communal violence killing thousands (I read that in Ramachandra Guha's "India After Gandhi").
Any other films out there that have had sparked something much bigger (I imagine) then the filmmakers and studios thought would happen?
Semi-current event: Mel Gibson's Braveheart became a significant catalyst for the recent Scottish Independence movement The movie the Exorcist played a role in the rise of American Charismatic Evangelicalism in the 70s and 80s which claimed that our society was possessed by demons. When you look at the witchcraft and Satanic Ritual Abuse scare of the 80s & 90s this film played a huge role in putting it in the public consciousness. Your question really depends on how you define impact. Return of the Jedi didn't necessarily inspire Reagan's "Star Wars" defense program, but he referenced it because it was something in the public consciousness.