After about one millennium during which it was a sovereign territory and then a distinct part of the Kingdom of France, Brittany disappeared as a political entity after the French revolution and was divided into 5 départements. When the administrative region of Brittany was created by the French government in 1956, the département of Loire-Atlantique (around Nantes) was left out and integrated into the Pays de Loire region.
This makes the Breton question a bit complicated because overall the Bretons would like their region to include all of Brittany while the successive governments have remained deaf to the request. The regions were reorganized in 2015, mainly to merge the 22 mainland ones into 13 larger ones, and not only Brittany wasn’t reunited but there was an unsuccessful push to merge it with others into a “Western France”.
The difficulty is compounded by internal questions, since Nantes is the economic powerhouse of Brittany (and the administrative capital of the Pays de Loire) while Rennes is the administrative capital, so some people could lose in breaking the status quo. Some French people, puzzled by the strong identity of the region, also fear that a reunification could lead to a drive toward autonomy (not unlike Scotland or Catalonia), a no-no in highly centralized France.
Not much happens though. Regions have small budgets (at €1,6 bn., Brittany’s budget is slightly higher than those of Nantes, Rennes, Loire Atlantique or Ille et Vilaine) and a limited scope of missions, and the state calls most of the shots anyway with a system of targeted subsidies. The Bretons work together when they feel like, the Breton culture being as vibrant in Loire-Atlantique as in the other Breton départements. There are attempts at merging organizations across regions to keep the Western France concept alive (with universities for instance), with mixed results at the moment. Protests and actions are regularly organized to promote the reunification and Bretons are waiting for the next opportunity.
Beyond the reunification issue, there are also questions about the protection and promotion of the Breton culture, language and identity, many individual and institutions in France still hesitating between nurturing the vibrant and diverse heritage of the French territories and conversely covering all of it under the mantle of a strictly French identity, symbolically and formally centered on Paris.