What are the major doctrinal differences between the American Catholic Church and the Roman Catholic church? Why did they split?
According to Wikipedia article on the American Catholic Church of the United States (ACCUS), if that is what you mean, it notes that there are quite a few differences between the ACCUS and the Roman Catholic Church.
I have a feeling that ACCUS is Catholic in a sense that it observes the seven sacraments, but it disagrees with the Roman Catholic Church on certain sexual-related issues, or at least the article appears to be focused that way. Call it a "liberal" version of the Roman Catholic Church, though any split-off group of the Roman Catholic Church would probably be recognized as Protestant despite the similarity of the theological traditions.