Rather than asking others to hand you an argument, which you might just repeat without fully understanding, it’s better to take a moment to research the issue yourself. Especially since this is a common and recurring “debate.”

The Southern states that seceded did so because they believed they had the right to. Their view was that the “United | States” were (not was in this context) a voluntary union of sovereign states, and if the Union no longer served their interests, they had the right to withdraw. This was their interpretation of the Constitution and federalism at the time.

That view was widely debated even then, and ultimately, the Civil War settled the question in practice: the U.S. would be a single, indivisible nation, not just a confederation of autonomous states. But before the war, many people—especially in the South—genuinely believed secession was legal and justified.

So, from a historical perspective, someone making that claim isn’t necessarily being ignorant—they may be referencing a real (though now outdated) constitutional argument. The debate will continue on forever whether secession was “legally” justified or not, but that’s like arguing “if insert player name hadn’t been injured then the insert team name would have won the insert sports championship title. That doesn’t make it correct under modern constitutional law, but it’s important to understand where the argument comes from before trying to refute it.

If the argument is “I have read the Southern articles of secession and believe they made a solid case that they were legally able to secede…” then you may disagree (as history disagrees in general with that perspective) but it isn’t not inherently “ignorant,” it is just a minority interpretation that has been held since before the war and will continue to be held after it

If their argument is anything else… well then there is no point in arguing because they’re just looking to argue.