You underestimate the strength of the 3rd Reich. It was something.
No country on Earth can hold its own if the rest of the world bands against it. If the US go ballistic, they’ll fall.
I’m more scared of whoever is in charge then taking the whole world with them using the big red button.
There is no general hard line, as everyone’s hard line differs. Rather than a hard line, the point of no return is likely defined by how many people consider their hard line crossed, and how many act on it.