Firstly, America is not nazi Germany 2.0, nazi Germany was America 2.0.
Secondly, Who will declare war on America?
China? They already are preparing for WW3 scenarios and Trump's statements do nothing to change the timeline of preparations
Russia? They are already at (proxy) war with the US
Iran or North korea? They have enough defensive capabilities to have a good chance of defeating a US invasion (which appears to be a matter of when not if). But no real offensive capabilities against the US (discounting US satelites).
India or NATO minus the US? Their ideological conviction would lead them to siding with American fascism rather than against it. Like, why would they invade the heartland of world-historical fascism when they themselves are fascist?
We just gonna sit around and let Nazi Germany 2.0 happen?
~~Modern america doesn't a tenth the (relative to the rest of the world) military or economic power that Nazi Germany did. And that's because~~ other countries haven't been sitting on their asses all this time. They've been struggling for decades to create the multipolar world.