As a European I do not know much about the American Civil War. We had about 1 page talking about it. We were basically thought: South seceded to keep their slaves because they were vital to their economy. Then the Union attacked them to free the slaves and reunify the USA. And that it was one of the first modern wars, foreshadowing the first World War.
But I often hear on the internet, especially from Southeners, that the South was justified in seceding from the Union and that the Union attacked them first. And that they were essentially the ""good"" guys.
How much truth is there to this?
You're highly unlikely to get a better answer than this one by /u/Georgy_K_Zhukov regarding the causes of the US Civil War. If I can sum up his post, in a lot of ways the war was the result of a lot of different issues. We could go on for pages discussing them, and he does. Ultimately though, "South seceded to keep their slaves because they were vital to their economy," is actually about as good a one sentence summary as you're likely to get.
As to who shot first? The South did. But there's actually an interesting discussion to be had there.
After the southern states started seceding, there was a discussion about federal facilities. The Union controlled a series of forts especially along the coasts. These had been built in response to the war of 1812 to defend cities and the like. Instead cities like Charleston discovered they could also act to blockade the cities. In particular Fort Sumter sat on an island in Charleston's harbor and totally dominated it. So long as the Union controlled the fort, it controlled the harbor. The first shots of the war were when supply ships headed to the fort were driven off. One of Lincoln's first acts was to raise a large supply convoy. To prevent it from reaching the fort, Sumter was attacked in earnest, and forced to capitulate. From a strictly legalistic point of view the south attacked the ship Star of the West and fired the first shots. Three months later they started shelling Fort Sumter with artillery and really fired the first shots intended to kill.
But at the same time the presence of an occupying force on South Carolina's soil functionally blockading one of the South's biggest ports, was a legitimate Casus Belli. If South Carolina was truly independent, well the fort just couldn't be reinforced and resupplied. On a high strategic level it couldn't be allowed to stay as it was.
Forcing the South's hand however proved to be a strategic victory for Lincoln. By forcing the South to either accept the fort or to shoot, he maneuvered them into a no win situation. Prior to the attack the North was very tepid toward the war. They didn't want the South to secede, but they weren't really willing to go to war over it. Once blood had been spilled public opinion hardened, and army recruitment climbed.