I can speak about europe, but I don't think that that the US started on a serious downward trend until nationalism started rising.
But let's say that the US was really on a downward trend -- it's still a big logical leap to say that it's because of some sort of lack of nationalism, especially because whatever downward trend there was has been sped up a lot with the rise of nationalism.
Nationalism of the sort we're seeing now is correlated with the collapse of nations, not with healthy ones.