Your argument about a standing army is not germane. The land on which Fort Sumter stood was the property of South Carolina. When South Carolina voted to secede, it declared its borders sovereign. The garrison at Fort Sumter then became an invading force whose presence could not be tolerated.
There is no provision in the Constitution forbidding secession. However, there IS one enumerating the powers of states where the Constitution does not explicitly assign those powers to the federal government. So if anyone was treasonous in the sense of ignoring constitutional constraints, it was the Union.
And as much as my “non-answer” disqualifies me as a further conversant, I have not resorted to name-calling or personal attacks on your integrity.
South Carolina deeded the fortress to the US in perpetuity. This is a historical fact. South Carolina had no intention of offering just compensation for the property that they seized.
You asked who started the Civil War. You got your answer: The traitors fired first on the United States Army and on United States territory. You tacitly admit this, by attempting to claim it didn't happen because "they declared it sovereign territory."
South Carolina had no legal basis for the claim that it had a "right to secede." There was a dispute about this between South Carolina and the United States. South Carolina ratified a document which put the adjudication of disputes between states and between states and the Federal government in the hands of the Supreme Court. South Carolina's refusal to abide by the Constitution of the United States -- with it ratified -- was a criminal act, and its claims of sovereignty are therefore a legal nullity at the time the traitors fired on Fort Sumter.