Towards the end of 1339 a new invasion was planned. The French ships and galleys assembled off the town of Sluys, in Flanders, and their crews solemnly vowed not to return to their own ports till they had taken one hundred English ships and five hundred English towns. In view of this invasion parliament was summoned in January 1340 "to adopt various measures relating to the navy." The sailors of the Cinque Ports undertook to have their ships ready, and in due course a fleet of two hundred vessels was formed, and more soldiers and archers assembled than could be employed. On his arrival on the coast of Flanders, Edward found that the various sections of his fleet had met, and discovered the French fleet of one hundred and ninety ships, manned by thirty-five thousand Normans and Genoese, lying at anchor off Sluys. The French fleet was in four divisions, their ships being fastened to each other by iron chains and cables. To the masts a small boat was suspended, filled with stones, which were to be hurled by the soldiers stationed on the tops. Trumpets and other martial instruments resounded from the French ships. The fight was long and fierce, for "the enemy defended themselves all that day and the night after." In one French ship alone four hundred dead bodies were found, the survivors leaping headlong into the sea. Only twenty-four of the French ships escaped, and no less than twenty-five thousand French and Genoese perished. The English loss was, perhaps, four thousand men, and all writers agree that it was one of the most sanguinary and desperate sea-fights recorded in the pages of history. Edward's modest letter regarding this victory is the earliest naval despatch in existence. Though the annihilation of the French fleet at Sluys did not surpass in importance the victory off Dover in the preceding century, it established the maritime supremacy of England.

To supply a covering force for the army which was besieging Calais in 1347 and to guard the Channel, England made a general demand for ships and seamen. The total number of ships mustered was seven hundred and ten; these were equipped with a full complement of fighting men. The "five Cinque Ports and two Ancient Towns," together with Seaford, Faversham, and Margate, contributed one hundred and five ships; London sent twenty-five ships, Fowey forty-seven, and Dunwich six. Three years later on August 29th, 1350, the battle known as "Lespagnols-sur-mer" was fought off Winchelsea, when Edward defeated a Spanish squadron of forty sail which had plundered several English ships, capturing twenty-six large vessels, the crews of which were put to death. This action firmly established the reputation of Edward III. as the King of England, whose name is more identified with the naval glory of England than that of any other sovereign up to the sixteenth century.

But reverses of fortune clouded the end of what had promised to be a glorious reign. In 1371 an engagement with the Flemings resulted in the capture of twenty-five ships by the English, but in June, 1372, the Spaniards completely defeated the English fleet of forty sail under the Earl of Pembroke off La Rochelle; the Spaniards not only having the advantage of size and numbers in their ships, but also in being provided with cannon, said to have been first used at sea in this battle. Immediately upon Edward's death an overwhelming fleet of French and Spanish ships swept the Channel, and Winchelsea, Rye, Hastings, Plymouth, Portsmouth and other ports suffered from the fury of the invaders.

The reign of Richard II. was redeemed from absolute barrenness in naval affairs by the victory of the Earl of Arundel in 1387. Taking advantage of the absence from England of John of Gaunt, Duke of Lancaster, who had sailed the year before to enforce his claim upon the crown of Spain, the French raised a powerful armament with a view to invading the British Isles. These preparations were made upon a most extensive scale, and were said to have included an army of a hundred thousand men and a fleet of ships which, if laid side by side, would have reached from Calais to Dover. The news of this terrible armament caused great excitement in England, and various preparations were made to receive it. The Earl of Arundel was made high admiral and was dispatched to sea with instructions to destroy the ships of the enemy as they disembarked; while the people on shore laid waste the country, and dealt with them as opportunity served. The winds and the waves, however, fought on England's side, and under stress of weather the army was disbanded and the enterprise abandoned. The Earl of Arundel, taking advantage of the situation, attacked the French fleet with great vigour, captured a hundred and sixty vessels, and proceeding to the Port of Sluys, destroyed the ships that had taken refuge there, and laid waste the country for ten leagues round.

The reign of Henry IV. was likewise characterised by abortive invasions on the part of the French. In 1403-4, La Marche, a young French prince, made a descent upon Falmouth with a view to helping Owen Glendower, the leader of the Welsh rebellion; but the attempt was an entire failure. In the spring of 1405, however, a second French fleet, consisting of a hundred and twenty sail and carrying large numbers of cavalry, bore down upon our southern coast. Once more our old allies, the winds and the waves, did us good service; for most of the horses fell victims to the rigours of the journey, and no sooner were the ships moored off Milford Haven than they were attacked by the squadron of the Cinque Ports, which burnt fifteen ships, captured six transports laden with food and ammunition, and cut off all supplies at sea. The French were rather more successful on land, but before the end of the year they were glad of an excuse for sailing back to France.

But the corporation of the Cinque Ports had practically fulfilled its purpose, and was now to give way to other organisations better adapted to the requirements of the times. Even at this early date some of the ports had begun to suffer from "the sea change," which eventually caused the majority of them to be deserted by the routes of commerce; and Henry V., finding that their harbours were no longer capable of building or sheltering the large ships which were required in his time, determined to establish a King's Royal Navy. So successful was he, that in his fleet which invaded France in 1415, and which consisted of one thousand four hundred vessels, carrying about six thousand men-at-arms and twenty-four thousand archers, were twenty-seven royal ships, some perhaps of the size of five hundred tons. After the return of Henry V. from the Battle of Agincourt, and during the negotiations which were to settle the relationships of England and France for the future, the Count of Armagnac, who had succeeded D'Albret, slain at Agincourt, as Constable of France, determined to attempt the recapture of Harfleur, held for the king by the Earl of Dorset, and with this view laid siege to the town by land, and sent the French fleet with a number of Genoese caracks and Spanish ships hired for the occasion to blockade the port from the sea. Henry V. in a great rage dispatched his brother, the Duke of Bedford, to deal with this formidable armament. The Duke assembled his ships at Rye in August 1416, and on the 14th of the same month reached the mouth of the Seine, at the head of a fleet said to number four hundred sail and to carry twenty thousand men. He found the Genoese galleys so tall that the largest of his ships could not reach to their upper decks by a spar's length, while the Spanish ships far out-matched his own for size and for the number of their crews. Notwithstanding the disparity of the forces the duke determined to attack the enemy on the following day; and on the morning of August 15th, 1416, taking advantage of the wind, he engaged the combined fleets with such vigour that he succeeded in capturing or destroying nearly five hundred ships, his men clambering up the Genoese galleys like so many squirrels and boarding them in gallant style. Having destroyed the fleet, the duke joined his forces with those of the garrison in repelling the attacks on land and sea, and compelled the Count of Armagnac to raise the siege and retire. The duke remained long enough to see the town placed in a state of defence and then returned to England.

In 1417 the Earl of Huntingdon being sent to sea with a strong squadron, met with the united fleets of France and Genoa, which he fought and defeated, though they were much superior to his—not only in number, but in the strength and size of their ships—taking the French admiral prisoner, and capturing four large Genoese ships, containing a quarter's pay for the whole navy.

The reign of Henry VI. added but little to the naval glory of England. In August 1457 a fleet fitted out in Normandy made a descent upon the coast of Kent and landed nearly two thousand men about two leagues from Sandwich, with instructions to attack the port by land while the fleet engaged it from the sea. In this case the English were taken by surprise, and the town pillaged and burnt, with great loss on both sides. Other attempts of the kind were also made at other parts of the coast. In the following year, Warwick, the King Maker, having been made admiral, caused several squadrons to be put to sea, to the officers of which he gave such instructions as he thought proper.

On Trinity Sunday, 1458, one of these squadrons fell in with the Spanish fleet and quickly came to hostilities; with the result that the English captured six ships laden with iron and other merchandise and destroyed twenty-six others. A year later Warwick himself put to sea from Calais with fourteen sail, when he encountered five large ships in the English Channel, three of which were Genoese and two Spanish, all of them being richly laden with merchandise. After an engagement which lasted two days he succeeded in capturing three of these, which were hauled into Calais, where their cargoes realised £10,000. It is said that in this engagement Warwick lost fifty men and the enemy nearly a thousand.

Jealous of the successes of Warwick, the French queen of Henry VI. sent Lord Rivers down to Sandwich to seek the assistance of the Cinque Ports in depriving the earl of the government of Calais; but when the ships were almost ready, Warwick sent a squadron under Sir John Dineham, which captured the whole fleet, carrying away Lord Rivers and Anthony Woodville, his son, who long remained prisoners in Calais. After this, one Sir Baldwin Talford undertook to burn the earl's fleet in the haven of Calais; this, however, proved but a vain vaunt. At last the Duke of Exeter, who had been made admiral, received information that the Earl of Warwick had set sail for Ireland, and stood out to sea to intercept him; the sailors in the king's ships, however, showed so much coldness in the cause, that it was not judged safe to risk an engagement, and Warwick, not wishing to destroy the king's fleet, passed by without molesting it. Later, Warwick, on an invitation from Kent, made a descent upon the country and encountered Sir Simon de Montfort, then warden of the Cinque Ports, with his squadron off Sandwich, which he attacked, defeated and destroyed, Sir Simon being killed in the engagement.