Actually there was, in the sense that was legal to have Roe vs Wade, Affirmative Action, use the Supreme Court and so on. It is not legal to break off from the United States and found your own nation from its borders, and also isn't constitutional to have segregation. What you are suggesting is a roll back, because you would have to change the constitution by putting an amendment in that makes segregation legal.
Your argument boils down to this: Abortion was illegal until the Supreme Court declared it to be legal. Secession is illegal. Well, other countries have dealt with secession in modern times. They find it legal. Canada was prepared to break up and put the question to their Supreme Court and a specific pathway was established.
The era of the Civil War has passed. The ties that bind us together as people have been frayed to mere strands by liberals. You argue that the moment a Somalian steps foot into the US, via legal immigration, that he and I have a bond to each other. We are countrymen. That's a fiction. That bond used to exist in the US but it's been severely weakened by the idiocy that liberals so love - multiculturalism - multiple cultures coinhabiting the same geographic space and governed by one set of laws. Following one set of laws doesn't form bonds of community. All ships on the sea follow Admiralty Laws but no one thinks that the crew on an American flagged vessel shares the same values as the crew on a Liberian flagged vessel.
This ultimately comes down to one question - what are YOU willing to do to stop ME? Are you going to take up arms, put your own life at risk, arrive at a battlefield and shoot it out with me? Multiply by X million. Will you bomb my cities, burn them down like Atlanta in order to keep the Union intact? Or are those days past us now?
Your argument boils down to this: Abortion was illegal until the Supreme Court declared it to be legal. Secession is illegal. Well, other countries have dealt with secession in modern times. They find it legal. Canada was prepared to break up and put the question to their Supreme Court and a specific pathway was established.
No it doesn't, it boils down to Supreme Court rulings determining that segregation is unconstitutional, and currently based on on the constitution and past behavior and conduct of the US government, it has been proven illegal to secede. You can't ignore the civil war as it happened, and precedents were set when the US government went to war with the Confederacy. It would have been different if the US government had accepted the Confederacy as an independent nation, but it didn't.
I don't argue multiculturalism is perfect, but just because a system has flaws doesn't mean its polar opposite is better. Some Somalians might really find it difficult, and not like America or fit in, but most would build a life here and become a part of the community. The problem with a multi-cultural system stems from uncontrolled immigration, lack of financial support and aid programs for the poorest members of society, and overall economic woes - and of course the fact that extremists come hand and hand with freedom of speech and expression.
But without multi-culturalism, you have a divided community, with one dominant ethnic group that has all the rights in a nation while others have second-class rights.The best case of a society that claims to be multi-cultural but actively discriminates against its citizens based on race, is Malaysia by treating non-malay ethnicities and Christians as second class citizens due to the Islamic beliefs of the Malay population and so on. That is ultimately what would happen if the world gave up on 'multi-culturalism', as there is literally no way to have a nation without a certain ethnic group without conducting ethnic cleansing or genocide - if you don't at least allow the people that aren't of that ethnic group to remain.
To answer your final question, yes I would (would need to learn to use a gun first though) - if any group decided it would just unilaterally secede without the approval of the Supreme Court and the US government. Just like I would if any state seceded without getting the approval of the Supreme Court, which would have to declare secession as constitutional, and then be supported in that judgement by the rest of the US government.
Basically you would have to fight the US military, and yes cities might be bombed by them, it is always a possibility, part of me wonders if militarization of the police isn't part of wider fears of rebellion or insurgency by groups in the United States, but that is for another thread.