European nations will need to increase the sizes and capabilities of their militaries because the US is no longer a reliable partner, whether or not Trump gets elected again. As Trumpism has come to define the Republican Party, the US is no longer a reliable partner.
Because it is the process of years and decades, European nations need to start building capabilities now, even if Trump loses the election. The US has demonstrated that they can move to a Russian-aligned power as the outcome of a single election, which was unthinkable ten years ago.
I’m still having trouble wrapping my head around that, but every “adults in the room will prevent this” has proven very flimsy, and there’s no reason that a post-Trump Republican Party wouldn’t be subjected to the same kind of politics.
The only saving grace is that, as far as I can tell, there’s no post-Trump plan and the republicans might split into two parties once he’s no longer among the living. If that does happen, they’re going to have trouble winning national elections. Trump is such a narcissistic egomaniac that he will not have a successor designated, so it will turn into an open field.