The first major battle of the hundred years war, certainly the first and most major naval battle of the entire war, Sluys saw the destruction by the English and the Flemish of a major portion of the French fleet which had been commanding the channel, harassing English shipping and raiding English costal towns since the beginning of the war. While the French would, in time, recover their naval power, it ensured that, for a while at least, the English coast and English shipping would be unimpeded by any action of the French fleet.
King Edward III of England, now able to make good on his standing desire to make an invasion of the continent with the help of his dearly bought Imperial allies, sailed from Orwell on 22 June, 1340, with a fleet of 200 ships, and made for the Flemish coast, where he was joined by his Admiral of the North Sea, Sir Robert Morley, with his fleet of 50 ships. These fleets, combined, sailed up the Flemish coast to Blankenberge where, on 23 June, Edward lay at anchor while sending Sir Reginald Cobham and Sir John Chandos ashore to meet with his Flemish allies and scout out the French position. They return with a wildly exaggerated report of 400 French ships packed into the mouth of the Zwin estuary. Edward III decides to postpone any action until the next day, when he can use the wind and the tide to his advantage.
The French fleet, variously estimated at 190 to 213 ships, was commanded by Hugues Quiéret, a Breton knight and Admiral of France, and Nicolas Béhuchet, a lawyer and treasurer for the King of France who showed a surprising naval talent, up to a point. Also in the fleet were a contingent of hired Genoese galleys, under the command of Egidio Bocanegra, or Barbavera. This fleet had, over the course of the previous year, been cruising the coast of England, attacking and burning towns, and generally causing havoc on the southern English coast. It had withdrawn to its ports over the winter, and had been reinforced by Philippe VI's decree of January 1340, commanding the creation of a 'Great Army of the Sea'. The principal section of this Great Army of the Sea had sailed from Harfleur on 26 May, towards Flanders, gathering further ships as it progressed. By the time it reached the Flemish coast it numbered some 202 ships, 6 galleys, 22 oared barges, 7 royal sailing ships and 167 requisitioned merchantman. On 8 June this fleet entered the mouth of the Zwin estuary and seized the island of Cadsand, then anchored between Cadsand and the town of Sluys.
While Edward III was meeting and scouting, the commanders of the French fleet conferred as to the plan of battle. Barbavera, the most experienced seaman of them, advised taking the fleet out of the Zwin, into open waters where they would have room to maneuver, but Béhuchet, who was in overall command, disregarded this advice, and remained anchored in the mouth of the channel, laid out in three great lines and chained together to form a theoretically impassable barrier. The cog Christopher, recently captured from the English, was in the front lines.
On the early afternoon of 24 June Edward and his fleet, a total of 250 ships, arranged in four squadrons, sailed down into the French lines. With the tide, the wind and the sun behind them they engaged the French at about 3 pm, pouring arrow fire down from the high castles of their ships onto the decks of the lower Frnch ships, and sending boarding parties over to engage in hand to hand fighting so fierce that Edward III himself is wounded in the thigh, some say by Admiral Behuchet, though this is unconfirmed, He was definitely wounded by an arrow or crossbow bolt at some point, but retained command of the action. One of the queen's ladies-in-waiting was killed in the action, which fairly clearly shows that every ship was involved in the battle.
By this time the French fleet had drifted eastward up against the Cadsand shore, allowing the English to attack the flank of the front line, and the admirals ordered the chains cast off as useless. By about 7pm the French front line had been broken through, and the English fell on the French second line of smaller ships. At this point, the Flemings, who had been watching from shore, came out of Sluys and other harbors and fell on the French fleet from the rear. As night fell the French third line, consisting mainly of the Normandy merchantmen, attempted to break out, and the fighting broke down into a series of skirmishes as the English tried to block their escape. By dark, the fighting had ceased, except for the Saint Jame of Dieppe, and a ship from Sandwich belonging to the Prior of Christchurch, which were unable to disentangle themselves. Of the 202 ships of the French fleet 190 were captured by the English, and a vast number of men killed, with credible estimates ranging from 16,000 and 18,000. Both of the French admirals were killed in the action Hughes Quieret when his ship was boarded, and Nicolas Behuchet hung from the mast of his ship after the battle, despite his having surrendered and been taken for ransom. It was thought by Edward III that the laws of chivalry didn't apply to the man who had been ravaging the English coastline for the last year. Barbavera managed to escape with the six galleys under his command, taking two English ships as prizes, and thirteen others made their escape in the early hours of the next morning, pursued by John Crabbe and the ships from Yarmoth.
Froissart's account of the battle may be read here