Which conflict was primarily about states' rights and slavery in the United States?

The conflict primarily about states' rights and slavery in the United States is the American Civil War. This war, which took place from 1861 to 1865, was fundamentally rooted in issues surrounding slavery and the desire of Southern states to assert their rights against what they perceived as federal overreach. Southern states argued for their right to maintain the institution of slavery as an essential part of their economy and social structure, and this led to significant tensions with Northern states that were increasingly opposing slavery.

The principle of states' rights was invoked by the Southern states as they sought to secede from the Union. They believed that individual states should have the authority to determine their own laws, including the legality of slavery. The culmination of these tensions and disagreements ultimately resulted in the Civil War, which directly addressed these central issues.

In contrast, the other conflicts mentioned do not center on the themes of states' rights and slavery to the same extent. The War of 1812 involved tensions over trade restrictions and maritime rights, World War I relates to international relations and alliances, and the Spanish-American War was focused on the United States’ expansionist policies and imperialism rather than internal divisions over slavery.