Historians who debate the origins of the American Civil War focus on the reasons that seven southern states (followed by four other states after the onset of the war) declared their secession from the United States (the Union) and united to form the Confederate States (simply known as the "Confederacy"), and the reasons that the North refused to let them go. Most of the debate is about the first question, the reason that some Southern states decided to secede. Most historians in the 21st century agree that conflict over slavery caused the war, but they disagree sharply on the aspects of this conflict (ideological, economic, political, or social) that were most important.