Why did the upper southern states secede from the Union and join the Confederate States of America after the attack on Fort Sumter?

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Explanation:

the answer is aggresion hope this and mark me as brainiest and add me as friend

well we know that it was fought between america and the united kingdom based on that i assume people consider the second war of independence because brtiain and england (uk)   the uk is made up of england and i assume thats why it was considered that