The Civil War (1861–1865) was significant for a variety of reasons.
First, it was America's bloodiest and most destructive war. The South lost 258,000 dead, and the North 360,000. Many of the dead died from disease or in prison camps. Both sides had predicted a short war, but the length of the war was one reason for its destructiveness. The South was devastated because most of the battles had occurred on its soil.
Second, the war decided the question of secession. Prior to the conflict, states like South Carolina had claimed that states had the right to leave the Union. The war decided that they did not have that right.
Next, the main cause of the war was slavery. The end of the war brought an end to that institution. The question of slavery had vexed the country since its inception.
There are present-day echoes of the war in the United States. Recently, there was a controversy over statues of Southern leaders on display. Many of them have since been removed from public display.