In the post-ww2 period we had a long period of people being anti-nationalist as a result of experience of what nationalism and this obsession with borders instead of people causes.
The current crop of liberals have no experience or connection to this and are incredibly easily led by the ultranationalists into supporting them, because nationalists share a priority with ultranationalists.
The primary issue here is nationalism. We need an absolutely massive anti-nationalism movement. Anti-nationalism is anti-fascism.