No they didn't. Ex Parte Milligan was a decision handed down by Chief Justice Taney from the Circuit Court bench in Baltimore. The entire Supreme Court never took up the issue.
No it wouldn't have. In fact, it would have been advisable for him to seek professional counsel prior to engaging in potentially illegal actions.
Yes it would. Constitutionally the court can only rule on matters which appear before them. They cannot issue advisory rulings on matters under consideration by the Executive or the Congress. In other words, they cannot tell Congress or the president than a certain action that they are planning is unconstitutional. That would violate the separation of powers.
Indeed, but nothing precludes their offering counsel on what their likely response to given proposed actions might be.
Separations of powers does. Congress legislates. For the court to say that they had better not pass a law because it might be unconstitutional interferes with their legislative powers.
Are the United States of America a sovereign nation? Or are we colonies of Great Britain? If we're a sovereign nation, why? And at what point did we become a sovereign nation?
We are sovereign nations because the other nations of the world recognize us as such and deal with us as such. We became a sovereign nation when they began to do so. Prior to that we may have considered ourselves sovereign, but Great Britain and the rest of the world considered us a rebellious colony. No other nation recognized confederate sovereignty. In the eyes of France and Great Britain and Russia and all rest of the world the confederacy was nothing more than a collection of U.S. states in rebellion.
Given that Lincoln didn't consider the confederacy another independent nation why in the world would he feel compelled to go to Congress for a declaration of war? Declare war on who? Ourselves?
So did President Bill Clinton, yet most here agreed he violated his oath of office a great many times, a great many ways, and deserved impeachment and removal from office.
What you and I think of the legality of Clinton's actions or Lincoln's actions are meaningless. Our opinions don't count, the Constitution counts. And the Constitution vests in the Supreme Court the awesome responsibility for interpreting that document. Not you. Not me. So point out to me a single instance where the Supreme Court ruled that Lincoln deliberately violated the Constitution or took an action that he knew to be wrong and I'll grant you that Lincoln acted in an unconstitutional and illegal manner.
The Confederate States of America had no problem with the United States of America. There was no threat of the South launching a conquering invasion of the North. The only threat to the Union was of other states breaking away from it.
The confederate states initiated an armed rebellion against the United States when they fired on the U.S. fort at Sumter. They started a bloody and costly war that eventually ended in their own self-destruction. The fact that you might have believed that the confederacy had not interest in conquest is meaningless, I doubt that the Japanese or the Germans seriously contemplated conquering the U.S. in World War II. But they started the hostilities in every bit the same way that the south did 80 years earlier.