Trump definitely has worked towards removing sanctions from Russia, and has several times promoted Russian 'peace deals' which would gift them chunks of Ukraine (an imperialist aim of Russia's). He can't do literally anything he wants, which is why this hasn't happened. His reasons for being a Russian ally go back decades to his longstanding business interests, which have been entwined with Russia for decades.

Also, my experience of nationalism in the global south suggests otherwise. Take Latin America, for example: nationalist movements may have been instrumental in developing support for independence from Spain, but actual independence from Spain was won by transnational movements not limited to any one state or ethnic identify. Moreover, the modern day nationalists in each state are basically racist movements which do nothing but undermine working class solidarity (Chilean nationalists, for example, are known to carry out acts of violence against anyone whose skin is dark enough that they might conceivably be Peruvian or Bolivian, despite the fact that this includes many Chileans with indigenous ancestry).