Regardless of Russia's less than amazing GDP Putin has been actively working to destablize Western Europe for a long while now. So, yeah, "Russia" might be the answer. Maybe Putin doesn't think he can dominate or control Europe like the form USSR may have dreamt of, but he can surely destabilize it leaving it no threat to his ambitions which may still be quite horrific (see his ongoing war in Ukraine, his illegal annexation of Crimea etc.)
It has been clear for some time that Trump does whatever it is that aligns with Putin's larger goals. Whether that is purely by random chance or what I cannot say. But pretty much every single thing Trump stands for (weakening NATO, dismissing our allies, insulting and belittling them etc.) goes right along with what Putin has expressed interest in doing for many years (even before Trump came to power).
Very good point. China will also dominate us economically as well. We've handed them the keys and we just hope they don't try to drive the car.