The Obvious Query About Lee And His Fellow Rebel Commanders:

No. Patriots.

"would you describe them as traitors?"

Yes.

"The ,,Spanish'' armies in the Latin American Wars of Independence were mostly composed of Latin Americans who had relatives in the insurgent armies."

And The German and Russian ones too.

Of course they were traitors. Just like Major Nidal Hassan, who decided his loyalty to Islam was greater than his oath as an Army officer & killed 13 fellow soldiers & wounded 30 more at Fort Hood, is a traitor. One who is currently on death row in Fort Leavenworth. The only difference is that the Confederate traitors killed far more than 13 of their fellow soldiers.

The Confederate States were not founded by or fought for by traitors. Those states made the formal decision among themselves to secede from the United States. Although the government of the U.S. wasn't pleased about it, to the Confederate States it was a legal and justified undertaking. There is no rational comparison between the formation of the C.S.A. and mass murder committed by some 21st century lunatic army officer in the name of Islam.

Interesting question.
The United States was a young country in 1861 when the Civil War started. Only 85 years after the Declaration of Independence. The new nation state was formed from previously separate political entities within the British Empire.
The new government was intended to manage interstate and foreign affairs, and not involve itself with matters within the states themselves. Moreover, even as a young nations, the US was vast and varied. Life in Maine was very different from life in Georgia or Texas. It was a nation of countries.

Think of the United Kingdom. It is a nation made up of the countries of England, Scotland Wales and Northern Ireland (once all Ireland). Were the Irish traitors for rebelling against London's rule in the early 1900's? Would Scottish people be traitors today if Scotland decided to leave the UK and it turned violent?

The USA is much more united today than it was in 1861. An "American" identity exists. It did not then.

I think where the Confederates fall flat is over the cause of their "rebellion." The enslavement of another human being is never defensible. I understand it was the norm at the time (in the South) and had existed in many societies in history. The "states rights" argument never held water because the only thing you couldn't do in Maine that you could in Georgia was own another human being.

In summation, Confederates were in the wrong, not because they were traitors, but because their actions supported the ownership of human beings.

Of course.

This isn't even a question.

They literally committed Treason.

Take up arms against the United States: There is zero question. It's right in Federal Law. Along with the punishment.