What are relatively forceful measures?
In essence, it involves the strong legal unification of the languages, scripts, customs, and religious beliefs of Spain's various regions, expelling all heretics that reject Spanish culture, and being prepared to battle with radical nationalists from the Catalonia Region and the Basque Region at any time.
If it were a later era, such methods would obviously be unfeasible. The nationalist sentiments would have already become deeply rooted, and support for regional autonomy and independence among Catalans and Basques would be widely embraced.
But in this era, even the concepts of Catalans and Basques are not that clear. Catalan farmers often can't even distinguish their own ethnicity; they only know they live in Catalonia. This is their only proof of being Catalan.
