Which war was fought over the secession of Southern states from the Union?

The American Civil War was fought primarily over the issues of slavery and states' rights, which culminated in the secession of several Southern states from the Union after the election of Abraham Lincoln in 1860. These states believed their rights were being infringed upon by the federal government, particularly regarding the legality of slavery in new territories and states. The conflict began in 1861 and lasted until 1865 as the seceded states formed the Confederacy and resisted the Union's efforts to restore them.

In contrast, the other wars listed do not involve the secession of states from a union. World War I and World War II were global conflicts with complex causes primarily centered around international alliances, imperialism, and national interests rather than internal divisions within a single nation. The War of 1812 was fought between the United States and Great Britain, primarily due to trade restrictions and maritime rights, and it did not involve state secession. Thus, the context of the question clearly aligns with the American Civil War as the correct answer, making it a pivotal moment in U.S. history related to secession.