movies

And he was against the civil war...

True enough in certain statements he made, but while admittedly he was no Alexander Stephens, he sure as shit got stuck in.

but back then America was more like the UN.

Not really. Regional identities were much stronger and enforcement mechanisms weaker, and no income tax meant the funds available to the Federal government were limited so it wasn't particularly flush with cash to throw its weight around, but the country had already withstood several constitutional crises that placed it as a polity WAY more integrated even than the modern EU, to say nothing of the UN. The degree to which secession was viewed as an inevitability and/or no big deal for the times is way overblown by people with a vested interest in Reconstruction going smoothly for the existing power structures, and then the lost cause types who inherited their narratives. Don't fall into their traps.

So when Lee’s state seceded, he saw no other option than joining the Souths military.

To be clear, this was also very much because he was a dedicated slaveowner, and not one known for treating enslaved people on his properties well, or for having any empathy with freedmen after. He had that certain old Virginia ambivalence about the institution itself, but he was absolutely convinced that whites owning blacks was "necessary" and by god he was good at doing what was "necessary."

Militarily, Lee was a skilled tactician (by contemporary US standards... European observers familiar with Crimea were aghast at the life-wasting Napoleonic nonsense they saw from both sides), but he had strategic blind spots that had him ultimately fighting a war the South was poorly suited for. Now, sure, he probably was the best General available at the start of the war, but not by so wide a margin as people like to claim, and personal lionization of Lee is another Lost Cause ratfuck.