This naval battle took place in the context of the Hundred Years' War between England and France. King Edward III wanted the County of Flanders as an ally, as it would form a good base for an English landing. However, the Count of Flanders was a vassal of King Philip VI of France. Flanders therefore did not want this, which changed when the wool export from England was halted. People such as Jacob van Artevelde then advocated a pro-English stance. In 1338 the time had come, and Philip VI wanted to suppress this alliance by force, causing the Flemish to turn even more against the French. The French punitive expedition, however, never materialised.
In principle, France was the stronger party and sent its fleet to Sluis to stop the English invasion. The French fleet was much stronger, but the English had more war experience due to their struggle against the Scots.
The French fleet, under the command of Nicolas Béhuchet, sailed towards Sluis, where the ships were anchored in three rows (lines) at the mouth of the Zwin in order to hold back the English who wanted to come ashore. As a result, however, no advantage could be drawn from the French superiority. It became not so much a naval battle as a man-to-man fight, in which 25,000 soldiers perished. When the French ships also caught fire, escape was no longer possible and the French lost the battle. In the process, 170 French ships were lost.
Source: Wikipedia