Final answer:
The three segments of the US when the Civil War broke out in 1861 were the Union States in the north, the Confederate States in the south, and the border states in the middle.
Step-by-step explanation:
The three segments of the US when the Civil War broke out in 1861 were the Union States in the north, the Confederate States in the south, and the border states in the middle. The Union States, also known as the Northern states, remained loyal to the United States and fought to restore the Union. The Confederate States, also known as the Southern states, seceded from the Union and fought for their independence. The border states, which included Virginia, North Carolina, Arkansas, and Tennessee, initially remained in the Union but later joined the Confederacy.