why did America have a civil over slavery?

Because when Lincoln won the election, the confederate states began to secede from the union, and then they attacked fort Sumpter. Until the attack on the fort, there was nothing illegal done, because there is no law saying a state that freely joined the union could not freely leave the union too. Lincoln used the attack on the fort to impose his will upon the southern states in order to preserve the union. That is why they had a civil war. Most of the soldiers of the south didn't own any slaves at all. They were fighting to keep the north in the north and out of their home states.

Some will say, State Rights. But, yes, that Right was to own another human being. The war had been brewing for some time. Why not a federal law? Laws are made by the Legislature & that legislature was controlled by the South (Democrats). It would have been impossible even to debate such a law. Only the rich landowners had slaves. Farmers, merchants & city folk didn't need them or even want them. Anyway, when the South broke away from the Union, the lost control of the House & Senate.  Otherwise, Lincoln would have been powerless to send troops into states (no president has the Constitutional power to do so). For 80 years, the problem existed & presidents could do nothing about it. Until the South rebelled & Lincoln sent troops to end it.

um..... the Civil War was about the supremacy of the Federal Government....

does that make it a bit clearer why they didn't just "pass a law?"

The federal government DID ban slavery. It was called the Emancipation Proclamation. The South did not want to adhere to the federal ban of slavery and so tried to secede from the Union and form a new country called the Confederate States of America in order to avoid having to conform to the federal ban on slavery. 

The federal government came back and told states that announced their secession that they didn't have the right to secede from the Union without permission of the Union. That's because each state when admitted to the Union, was admitted by the Union passing a law through Congress and signed by the President that made each state a state, so their membership was by law, meaning a law would have to be passed by the Union in order for a state to no longer be a member of the Union, in order for a state to secede. The South, the states that would make up the CSU, didn't have the votes, however, to get laws approving their secession passed by Congress, which the Union knew, much less override the veto Lincoln would most surely impose. 

Knowing this, the South tried to legally overcome the obstacle by arguing that the Articles of Confederation, the founding document of the United States of America that preceded the Constitution, expressly gave a state the right to secede by a simple majority vote of the voters within the state, which was true, so all a state needed to do was put a referendum to the voters of said state, and if it passed with a simple majority, that constituted secession. And that's exactly what they did. The states in the South put a referendum to voters and each state that seceded did so having gotten a majority of voters within the state to approve secession from the United States. 

The United States government, however, asserted that the referendums were moot because the Articles of Confederation were no longer in effect as they had been repealed and replaced by the US Constitution, whose express purpose was to strengthen the federal government, to empower the federal government to tax and govern over states, and which did not create any such avenue for secession, Lincoln arguing that the United States, while founded by the Articles of Confederation as a confederacy, a union in which state membership is at-will by each state, ended up under the Constitution as a federation, a union in which state membership is legally mandatory by the passage of law after admittance and secession can only occur by the passage of law by said union that allows it.  

The South then argued that that was a matter of opinion as it is not expressly stated in the US Constitution.

The United States government came back and said, that while it is not expressly stated in the US Constitution, the documents that joined each of their states to the United States, that admitted them to the union, do expressly make them members of the Union, and without overturning those laws by passing new laws, they stood and the states remained states of the United States by law, similar to how a legal marriage isn't dissolved just because you decide to leave but requires going through legal procedure to undo the legal procedure that contractually and by law binds you in that union. 

The South was out of arguments and had had it. Come hell or high water, they were going to keep their slaves. So, the South tried to secede anyway to avoid having to follow the US government ban on slavery, stating any imposed enforcement by the US government, which they claimed they no longer recognized, would be met with military force, thusly creating a Confederate States of America in order to combine their strength and me a more formidable adversary should the United States try to call them on their threat of military force, formidable enough that they'd hoped the United States government would be dissuaded from challenging them on secession. 

Lincoln didn't blink, and US government had no choice but to use military force to force the issue in order to maintain the Union. Thusly, the US Civil War began.

the @Anon  idiot below is obviously an ignorant jilted rep, who doesn't know shiit

https://www.ducksters.com/history/civil_war/causes...

why essentially did they just not do what you suggest....because some people cant take no or even a compromise for an answer it has to be thier way or else, well they got there butts well kicked, so justice was served..

It wasnt over slavery. Get an education away from common core.