Henry's piety and his love of order and orthodoxy were a source of much trouble to the unhappy Lollards. From the moment of his accession he bore very hardly upon them, and redoubled the severity of the persecution which his father had begun. He did not spare even his own friends, but arrested for heresy Sir John Oldcastle, Lord Cobham, who had been one of his most trusted servants. When accused of holding the doctrines of Wicliffe, Oldcastle boldly avowed his sympathy for them, spoke scornfully of the Papacy and its claims, and taunted his judge, Archbishop Arundel, with all the sins and failings of the clergy. He was condemned to be burnt, but escaped from the Tower and hid himself in the Marches of Wales. Long afterwards he was retaken, and suffered bravely for his opinions.

Henry's ill-treatment of the Lollards drove the unfortunate sectaries to despair. Some of the more reckless of them planned to put an end to their sufferings, by seizing the king's person, and compelling him to relax the persecution. They tried to stir up a popular rising, like that of Wat Tyler, but Henry got timely notice of their plot. When they began to assemble by night in St. Martin's fields, outside the gates of London, he came suddenly upon them with a great body of horse, and scattered them all. Forty were hung next day as traitors, and for the future they were treated as guilty of treason as well as of heresy.

Fortunately for England, Henry had other things in his mind besides the suppression of the Wicliffites. He knew that nothing serves so well to quiet down internal troubles as a successful and glorious foreign war. He believed himself, and rightly, to be capable of leading the national forces to victory, and he knew that England's old neighbour and enemy across the Channel was weak and divided. Accordingly, from the moment of his accession Henry began to prepare for an assault on France. He was determined to claim not merely the restoration of the lost provinces of Guienne, but the crown of France itself, as Edward III. had done in the days before the treaty of Bretigny. It is hard to discover how a sincerely religious and right-minded man, for such Henry of Monmouth undoubtedly was, could persuade his conscience that it was permissible to vamp up once more these antiquated claims. It would seem that he regarded himself as a divinely appointed guardian of law, order, morality, and religion, and had come to look upon the French factions with their open wickedness, their treason, treachery, murder, and rapine, as emissaries of Satan handed over to him for punishment. Moreover, Henry was, as we have said, a very zealous servant of the Church, and the Church did its best to egg him on to the war. Chicheley, the Archbishop of Canterbury, was one of the chief supporters of it, partly because he wished to distract attention from the persecution of the Lollards, and partly because Parliament had been talking of a proposal to confiscate some Church land, and the archbishop thought that he had better give them some other and more exciting subject of discussion. In his old age, Chicheley bitterly regretted his advice to King Henry, and built his college of All Souls at Oxford, to pray for the repose of those who had fallen in the great war which he had set going.

Before he had been a year upon the throne, Henry had broken with France. It was in vain that the Dauphin and the Armagnac faction, who were at this time predominant, endeavoured to turn him from his purpose. They offered him the hand of the Princess Catherine, the daughter of their mad king Charles VI., and with her the lost provinces of Aquitaine and a dowry of 600,000 gold crowns. But Henry only replied by asking for all that his ancestors had ever held in France, the ancient realm of Henry II., extending from Normandy to the Pyrenees. When this preposterous demand was refused, he summoned Parliament and laid before it his scheme for an invasion of France. The proposal was received with enthusiasm, partly from old national jealousy, partly because the English resented the doings of the French in the time of Henry IV., when Norman privateers had vexed the Channel ports, and French succour had been lent to Owen Glyndower and the Scots. The Commons and the clergy gave the king very liberal grants of money, which he increased by seizing the estates of the "alien priories," that is, the religious houses that were mere branches and dependencies of continental abbeys.

By spending every shilling that he could raise, and even pawning the crown jewels, the king collected and equipped a considerable army. He assembled at Southampton some 2500 men-at-arms and 7000 archers for the invasion. Just before he embarked, however, he found himself exposed to a deadly peril, which showed him how precarious was the hold of the Lancastrian dynasty on the throne. A plot had been formed by his cousin, Richard of Cambridge, the younger brother of that Edmund of Rutland who betrayed the rebels of 1399. It had as its object the murder of Henry and the coronation of Edmund, Earl of March, whose sister Richard had married. In the plot were implicated Lord Scrope, a kinsman of the archbishop whom Henry IV. had executed and several others who had grievances against the house of Lancaster. The king sent them all to the block, and would not delay his sailing for a moment.

He landed in Normandy late in the summer of 1415, and laid siege to Harfleur, which then occupied the position that Havre enjoys to-day, and was the chief commercial port at the mouth of the Seine. On the news of Henry's approach, the French factions for once suspended their hostilities, and many of the Burgundians, though not Duke John himself, agreed to assist the Armagnacs in repelling the invaders. But they were so long in gathering that Harfleur fell, after five weeks of siege. The capture, however, had cost the English dear; not only had they lost many men in the trenches, but a pestilence had broken out among them, and a third of the army were down with camp-fever. After shipping off his sick to Southampton, and providing a strong garrison for Harfleur, King Henry found that he had no more than 6000 men left, with whom to take the field against the oncoming French. But he would not withdraw ingloriously by sea, and resolved to march home to Calais across Northern France. This enterprise savoured of rashness, for the whole countryside was swarming with the levies of the enemy. They had placed the Constable of France, John d'Albret, in command: with him were the young Duke of Orleans and all the rest of the Armagnac leaders. Anthony of Brabant, brother to the Duke of Burgundy, was hurrying to their aid from the north. By rapid movements—his whole army, archers as well as men-at-arms, had been provided with horses taken from the countryside—Henry reached the Somme. But he lost time in trying to force a passage, and when at last he crossed the river high up, near Peronne, the Constable and his host had outmarched him and thrown themselves across the road to Calais. They were at least 30,000 strong, five times the force that Henry could put in line, and were in excellent condition, while the English were worn out by their long travel, amid violent October rains, and over bad country crossroads.

When King Henry reached Agincourt, he found the French army drawn up across his path, and was forced to halt. The Constable, like King John at Poictiers, was confident that he had the English in a trap, for they had exhausted all their provisions, and had the flooded Somme in their rear. Henry, however, was determined to fight, and put his hope in the bad management which always characterized the disorderly armies of feudal France. He was not disappointed: the Constable dismounted all his knights and bade them fight on foot, for fear of the effect of the archery on their horses. Only a few hundred mounted men formed a forlorn hope in front. He arranged his army in three heavy columns, one behind another, and formed the front entirely of mailed men-at-arms; the cross-bowmen and light troops were placed in the rear, where they could be of no possible use. The week had been rainy, and the space in front of the French was a newly ploughed field sodden with water, and hemmed in with woods and villages on either hand. At its further end the English were waiting. Henry had drawn them up in a single four-deep line, in order to make a front equal to that of the enemy. So arranged they just filled the space between the woods. The archers were on the wings, protected by chevaux-de-frise of pointed stakes which they had planted in front. The king with his men-at-arms formed the centre; a small flanking force of archers had also been sent into the woods on the right.

The Constable led his men straight on the English front, but they had a mile to go across the greasy mud of the fields. To men arrayed in the full knightly panoply, which had vastly increased in weight since the days of Edward III., the ploughland was almost impassable. After a space they began to sink as far as their ankles, and presently as far as their knees, in the mud. The mounted men struggled on, and gradually drew near the English, but they were shot down one after another as they slowly forced themselves up to the stakes of the archery. The main body of the first column never won its way so far; it literally stuck fast in the tenacious clay and stood a few score yards from the English line, as a target into which the archers emptied whole sheaves of arrows. The crowded mass was soon full of dead and dying, for at such short range armour could not protect its wearers. The whole column reeled and wavered. Then King Henry, seeing the moment was come, bade his whole line charge. The lightly equipped archers could cross with ease the ploughland where the men-at-arms had found themselves unable to move. They flung themselves upon the French knights, and by the force and fury of their assault completely rolled them over. Though unprotected by mail, they obtained a complete ascendency over the enemy, dashing them down with their axes and maces till they lay in heaps two or three deep. Henry and the band of men-at-arms around him seem to have met with the only stubborn resistance: the king had to fight hard for his life, and was nearly slain by the Duke of Alençon, who had already struck down his younger brother Humphrey, Duke of Gloucester. Alençon, however, was slain, and after his fall the whole of his column was destroyed or captured.

Without a moment's hesitation, the English pushed on to attack the second column, which was slowly advancing through the mud to aid the van. Incredible as it may appear, their second charge was as successful as the first, though the victors were exhausted and thinned in numbers by the previous fighting, and did not muster half their adversaries' force. Just after he had routed this second column, Henry received an alarm that a detached body of the French had assailed his camp in the rear, and were coming up to surround him. He at once bade his men slay the prisoners they had taken, a harsh and, as it proved, an unnecessary order, for the French in the rear only plundered the camp, and then dispersed with their booty. Although the king had completely scattered or destroyed the second French column, the third still remained in order before him; but, cowed by the fate of their comrades, they turned and retired hastily from the field, though they should by themselves have been more than enough to overwhelm the exhausted band of English.

In this astonishing victory, Henry's small army had slain a much larger number of men than they mustered in their own ranks. The Constable of France, Anthony, Duke of Brabant—brother of John of Burgundy—the Dukes of Bar and Alençon, and a whole crowd of counts and barons, had fallen; it is said that no less than 10,000 French were slain, of whom more than 8000 were men of gentle blood. In spite of the massacre of captives in the midst of the fighting, there were still some prisoners surviving. They included the young Duke of Orleans—the titular head of the Armagnac faction—the Duke of Bourbon, the Counts of Eu and Vendôme, and 1500 knights and nobles more. The English in this terrible fight had lost less than 200 men, but among them were two great peers, the Duke of York—the Edward of Rutland of whom we read in 1399—and the Earl of Suffolk.

Henry retraced his way to Calais, and crossed to England with his prisoners and his booty, there to be received with splendid festivities by his people, who regarded the glory of Agincourt as a sufficient compensation for the losses of a costly campaign which had added nothing save the single town of Harfleur to the possessions of the English crown. The ransoms of a host of noble captives were relied upon to replenish the exchequer, and the fearful losses of the Armagnac party, who saw half their leaders slain at Agincourt, would evidently weaken the strength of France in the remainder of the war.

Henry did not cross the Channel again in the year 1416, which he spent partly in negotiations with the Duke of Burgundy, whose help he wished to secure against the Armagnacs, partly in treating with the Emperor Sigismund about the common welfare of Christendom. Sigismund was hard at work endeavouring to put an end to the "Great Schism," the scandalous breach in the unity of the Church caused by the misconduct of the rival Popes at Rome and Avignon. He visited England, and won Henry's aid for his plans, which brought about the reunion of Christendom at the Council of Constance—a reunion under evil auspices, since it was marked by the burning of the great Bohemian teacher John Huss, who had made the doctrines of Wicliffe popular among his Slavonic countrymen in the far East. Moreover, it restored the unity of Christendom, but did not reform either the papacy or the national Churches. As this was not done, the general outbreak of religious ferment was made inevitable in a later generation; after the failure at Constance to reform the Church from within, it became necessary to reform her from without.

Having come to an agreement with the Duke of Burgundy, and obtained from him a promise of neutrality, Henry invaded France for the second time in the summer of 1417. He took with him an army of somewhat over 16,000 men, landed in Normandy, and began to reduce one after another all the fortresses of that province. Utterly humbled by the memory of Agincourt, the Armagnacs made no attempt to meet him in the open field. Some of the Norman towns held out gallantly enough, but they got no aid from without. At the end of a year the whole duchy, save its capital, the city of Rouen, was in English hands. Henry then assumed the state of Duke of Normandy, and put the whole land under orderly government, a boon it had not enjoyed for twenty years. He gave Norman baronies and earldoms to many of his English followers, and handed over the control of the cities to burghers of the Burgundian faction, who served the English readily enough, out of their hatred for the Armagnacs. For thirty years Normandy was to remain English. Rouen was added to the rest of the duchy after a long siege of six months, in which half the population perished by hunger. Irritated by this long resistance, Henry imposed on it the harsh terms of a ransom of 300,000 crowns, and hung Alain Blanchart, the citizen who had been the soul of the obstinate defence (January, 1419).

While the conquest of Normandy was in progress, the French factions had been more bitterly at strife than ever. In 1418 the Burgundian party in Paris rose against their rivals, and massacred every man on whom they could lay hands, including Bernard of Armagnac himself. The control of the party of the feudal noblesse then passed into the hands of the young dauphin Charles, the heir of France.

The fall of Rouen, however, frightened John of Burgundy, and unwilling that France should fall wholly into the power of his ally King Henry, he made proposals for a reconciliation with the Dauphin and his Armagnac followers. The treacherous young prince accepted the overtures with apparent cordiality, and invited Duke John to meet him on the bridge of Montereau to settle terms of peace. But when Burgundy came to the conference, he was deliberately slain by the Armagnac captains, in the presence and with the consent of the Dauphin (August, 1419).

The murder of Montereau was destined to make Henry master of France. When Philip of Burgundy, the son of Duke John, heard of his father's death, he vowed unending war against the Dauphin and his faction, and took the field to help the English to complete the conquest of France. Nor was Philip of Burgundy the only helper that Henry secured: the Queen of France, Isabella of Bavaria, bitterly hated her son the Dauphin, and was glad to do him an evil turn. She proposed that Charles should be disinherited, and that the crown should pass with her favourite daughter Catherine to the hands of the English king. So at Troyes, in Champagne, Henry, Philip of Burgundy, and Queen Isabella concluded a formal treaty by which Henry received Catherine to wife, and was to succeed to the French throne on the death of his father-in-law, the old King Charles VI., who still lingered on in complete imbecility (June 2, 1420).

The treaty of Troyes put Paris and the greater part of Northern France into Henry's hands. Casting national feeling aside in their bitter partisan spirit, the Burgundian faction everywhere accepted the King of England as the lawful regent and governor of France. South of the Loire the Dauphin and his Armagnac friends still held their own, but north of it they only possessed scattered fortresses dotted about in Picardy, the Isle-de-France, and Champagne, from Boulogne in the north to Orleans in the south.

After taking formal possession of Paris and holding a great meeting of the Estates of the French realm at Rouen, Henry returned in triumph to England with his young wife. He had reached a pitch of success in war such as no English king had ever attained before, and the nation, blinded by the personal merits of its king and gorged with the plunder of France, forgave him all his faults. The waste of life and money, the never-ending persecution of the Lollards, the precarious tenure of the conquests in France—due, in sober truth, merely to the aid of the Burgundian faction—were all forgotten.

Henry had not long been in England, when bad news crossed the Channel after him. He had left his brother Thomas, Duke of Clarence, with a small army, to hold Maine against the Dauphin's adherents. But the Armagnac bands had lately been strengthened by succours from Scotland, under the Earl of Buchan, the son of the regent Albany. For, although the King of Scots had been a prisoner in English hands for ten years and more, his subjects and his uncle the regent were not thereby constrained to keep the peace with England. Pushing forward rashly to attack the Scots and Armagnacs, Clarence was routed and slain at Beaugé (1421). The enemy at once overran Maine, and began to infest the borders of Normandy.

This compelled the king to cross once more over the sea in order to repair his brother's disastrous defeat. In a campaign extending from the summer of 1421 to that of the following year, he cleared the Dauphin's army out of their foothold north of the Loire, and then proceeded to starve out one by one their isolated strongholds in the north of France, the chief of which were Dreux and Meaux.

It was during the siege of Meaux, which continued all the winter of 1421 and spring of 1422, that Henry's health began to give dangerous signs of breaking up. He had been campaigning from his boyhood, and had never hitherto shown any weakness of constitution. But the winter colds of 1421-2, or the camp-fever bred in the trenches during the long siege of Meaux, had brought him very low. He was carried back toward Paris in a desperate state of weakness from ague and dysentery. Soon after, to the horror and dismay of the English and their French partisans, he died at the castle of Vincennes on August 31, 1422, before he had attained his thirty-fifth year.