I think he's been pretty consistent on the issue of European defense spending, actually. In any case, if Trump is inconsistent, that itself can be considered a warning -- that sometimes the US elects inconsistent presidents.
>I also think such an abrupt change of mind from the US hurts both sides and nobody earns anything from it.
Nothing short of this "abrupt change" has been successful in getting Europe to take responsibility for its own security. As I said, US presidents have been complaining about this for ages. I hope you finally listen, because my enthusiasm for defending Europe (as an American and a registered Democrat even) is currently quite low: https://news.ycombinator.com/item?id=43459774
>nuclear umbrella from France towards other European countries
We'll see if France actually commits to that, or if it is all talk. If Nation A extends its nuclear umbrella over Nation B, that basically means that Nation A is putting its own cities at risk in case of nuclear retaliation, for the sake of Nation B. As an American, it sort of boggled my mind when I learned that we were putting our own cities at risk for the sake of our allies this way.
And it seems that Europe's way of thanking us is to make fun of our healthcare system, accuse us of the non-crime of "cultural imperialism", etc... See what I'm saying about enthusiasm to defend you guys? Something needs to change in the relationship.