Jingoistic nationalism as espoused by the likes of Messrs. Trump and Abbott1 threatens humanity and its environment because it assumes the existence of an Other.
The existence of an Other creates an "us versus them" axiom, which logically leads to solutions that divide.
However, the major threats to humans and our environment this century (climate change, mass migration, globalising economy, pandemics etc) require integrated systemic solutions. And to reach integrated systemic solutions, one cannot subset a part of the system into an Other.
1. Tony, the former Prime Minister of Australia.