Union troops: Soldiers who fought for the Union during the American Civil War. They aimed to preserve the United States and abolish slavery.
Secession: The act of withdrawing or separating from an organization or government. In this context, it refers to Southern states leaving (or attempting to leave) the United States.
Civil War: A war fought between groups within one country. In this case, it was fought between Northern and Southern states in America over issues including slavery and states' rights.