War between the French and Spanish crowns, the two Grand Powers of the sixteenth century, had important consequences in shaping collective identities and cross-border relations of frontier communities of both monarchies. That was the case of the Basque maritime provinces of Guipúzcoa in Spain, and Labourd in France. The hostility between these two frontier communities was not limited to the borderland region but in fact extended to the high-sea and beyond. The conflict embraced the Atlantic, not only in European waters but also in distant places like the Spanish-American colonies and especially the important Newfoundland fisheries. My research on the frontier-like confrontation between Guipúzcoa and Labourd corsairs/fishermen at home and in the rich fisheries of Newfoundland is relevant to explain progressive decay of the Spanish maritime policy, and so the ascension of the English and French in North-America. In this sense the study of judicial trials and local “peace treatises” (tratados de buena correspondencia, lies et passeries or lines of amity) between maritime communities constitues a perfect source to analyze the link between “public” and “private” warfare and the interplay between global strategy and local interests on important matters for both the crown and the communities such as fisheries, course, commerce and the navy. The aim of this project is to go further in the comprehension on the use of royal institutions and military apparatus by local communities in order to implement their own power strategies in relation with rival communities across the border and to what extent were royal authorities, both in the territory and the court, conscious of this fact. In the same fashion it is very important to understand the king’s interests at local level as well as his concern on political, social and economic stability in borderlands.