What event started the Civil War?

No one event stated . Officially the war began with the shelling of Fort Sumter in Charleston Harbor.

Until the confederate force of South Carolina open fired on Fort Sumter there was not an open war between the north and the south.

In a sense the war started with the declaration of succession by South Carolina followed by other slave states.

Open warfare between the pro slavery forces and pro free state forces had already started in Kansas.

The nullification of by northern free states of the fugitive slave act could be said to have started the war.

The nullification by South Carolina of the tariffs of 1828 was a from the union by South Carolina. The Tariffs were later repealed but if the tariffs had not been repealed that could have been the beginning of open warfare.