Today, the political left considers the rebels who participated in the American Civil War—also known as the War Between the States—as traitors.
My opinion is that people think this way because they are not true Americans; they are imperialists. They believe in the right to rule and own other people.
Our country is different from most others in the world in that sovereignty does not rest in a king or a government, but in the people. In our country, the people are sovereign.
In the United States, the people are organized in two ways: by states and as a collection of states forming a nation. The nation itself is sovereign. The government of the United States is sovereign because it derives its sovereignty from the people.
Likewise, the people of each state are sovereign, and the state governments are sovereign because they also derive their authority from the people.
I believe—though I’m likely in the minority—that the treason clause in the Constitution refers to individuals, not states. After all, the Founders did the same thing with the Declaration of Independence, and I don't believe they saw themselves as traitors. For the Declaration to be accepted, it had to be ratified by a majority of the states—each a sovereign unit. States cannot commit treason because they are sovereign, and the Founders, acting as agents of those sovereign states, were not traitors. I don't think the Founders saw themselves as traitors.
Likewise, the individuals who voted for secession were not traitors, because as a collective—acting as sovereign states—they were exercising a sovereign right.
You can’t be a traitor against yourself.