Which war ended with significant changes to U.S. territory and governance?

The Civil War resulted in significant changes to U.S. territory and governance, primarily because it led to the preservation of the Union and the abolition of slavery, which transformed the legal and social framework of the country. The aftermath of the Civil War brought about the Reconstruction era, during which federal policies were implemented to reintegrate the Southern states into the Union and to address the rights of newly freed African Americans. Additionally, the war caused shifts in power dynamics, prompting legislative changes such as the 13th, 14th, and 15th Amendments to the Constitution that fundamentally altered the governance of the nation by ensuring civil rights and voting rights for all citizens, regardless of race.

The American Revolution did result in significant territorial changes, but it was primarily about independence from British rule rather than a change in governance within an existing national framework. The War of 1812 had some territorial implications, but it did not lead to as profound a shift in governance as the Civil War. The Spanish-American War did result in the acquisition of territories such as Puerto Rico, Guam, and the Philippines, which expanded U.S. holdings globally but did not invoke the same level of internal governance changes as the Civil War. Thus, the Civil War's impact on both