[AFAIK]

Many international borders are fluid and populations often overlap one another. Southern Vietnam used to be a province of Cambodia before which it was part of the nation of Champa. Myanmar is made up of multiple ethnic groups. Kashmir has a mixed population.

How would fascists forge and maintain distinct ethno-religious identities within borderlands?
What policies might be implemented towards stateless groups such as the Roma in Europe or the Rohingya in Myanmar?

[Red_Bull]

This is why fascism has to be based in something concrete like ethnicity and language. If someone tells you they're a Belgian Nationalist you would die from laughter. Half the nations on earth have no basis for existing.

[Wittmann]

How would fascists forge and maintain distinct ethno-religious identities within borderlands?

As the above poster also mentioned, Fascism needs a concrete and definable basis. An "American Fascist Party" would be laughable as well, and would most likely end up being "Western European American Fascists", you need a common element to unite your nation.

I'd probably begin a process of long-term deportation, or offer relocation to border-regions or colonies. It's all situational really...