France developed into a nation-state through a combination of political, social, and cultural transformations over several centuries. Initially, the Capetian dynasty in the medieval period played a key role in unification, establishing a more centralized monarchy. The marriage alliances between monarchs did help to expand territories and consolidate power, but they were not the sole factor in the development of France as a nation-state.
The adoption of an official language, specifically French, along with the promotion of Catholicism as the dominant religion, contributed significantly to creating a shared national identity among the diverse populations of the kingdom. This linguistic and religious unity fostered a sense of belonging and cohesion amongst the people.
Furthermore, the end of the Hundred Years' War in the 15th century marked a critical turning point in solidifying France as a centralized nation-state, as it led to the end of English claims on French territories and reinforced a sense of national identity.
Additionally, the Renaissance and the Reformation prompted new ideas about governance, identity, and society, further aiding the development of a cohesive national identity.
In summary, France's evolution into a nation-state was a multifaceted process involving marriage alliances, the establishment of an official language and religion, the impact of military conflicts, and the cultural shifts of the Renaissance and Reformation.