He received many new properties: Hinton and Spaine in Berkshire; Great Haseley, Ascott, Deddington, Pyrton and Kirtlington in Oxfordshire; Wincomb in Buckinghamshire; Longbenington in Lincolnshire; Kneesol in Nottinghamshire; Newnsham in Gloucestershire, Wix in Essex, and Bosham in Sussex

He was made Earl of Northampton in 1337, one of the six earls created by Edward III to renew the ranks of the higher nobility. Since de Bohun was a younger son, and did not have an income suitable to his rank, he was given an annuity until suitable estates could be found.

De Bohun accompanied the King to Flanders due to the English Channel constantly being attacked.

An early winter forced a pause in the Channel warfare, and 1339 saw a vastly different situation, as English towns had taken the initiative over the winter and prepared organised militias to drive off raiders more interested in plunder than set-piece battles. Responsibility over these militias was placed in the hands of several leading Earls, who were warned that if they failed to defend their stretch of coastline there would be penalties. Although piracy at sea was still a serious problem, with ships burnt and crews massacred as far north as the Bristol Channel, the large scale raids of 1338 were over. An attack on Jersey failed as the island was now too strongly defended and attacks on Harwich, Southampton again and Plymouth were driven off with heavy losses, the mercenary elements of the French force unwilling to risk a large scale battle. Hastings was burnt to the ground, but it was little more than a fishing village at the time and did not represent a major success. The combined fleet was reduced to attacking fishing boats and parading the bodies through the streets of Calais.

An English fleet had also been constituted over the winter and this was used in an effort to gain revenge on the French by attacking coastal shipping. The result was an embarrassing disaster as the mercenary captains of the fleet realised that more money could be made by attacking and looting the Flemish convoys of Edward's allies rather than the French, forcing Edward to pay a huge amount of compensation and endure severe diplomatic embarrassment. This force did prove vital though in July, when 67 French and mercenary vessels attempted to attack the Cinque Ports. The expedition was met by organised militia at Sandwich and turned towards Rye, burning several small villages on the way but failing to land at the town. There the English fleet under Robert Morley caught up with them, forcing the French force to flee back across the Channel. This scare had been too much for the Genoese mercenaries who made up the most experienced part of the French fleet, and they demanded more pay. King Philip VI responded by imprisoning fifteen of them, whereupon the others simply returned to Italy, at a stroke costing the French their best sailors and ships as well as two thirds of their navy.

In the early years of the war, Edward III allied with the nobles of the Low Countries and the burghers of Flanders, but after two campaigns where nothing was achieved, the alliance fell apart in 1340. The payments of subsidies to the German princes and the costs of maintaining an army abroad dragged the English government into bankruptcy, heavily damaging Edward’s prestige. At sea, France enjoyed supremacy for some time, through the use of Genoese ships and crews. Several towns on the English coast were sacked, some repeatedly. This caused fear and disruption along the English coast. There was a constant fear during this part of the war that the French would invade. France's sea power led to economic disruptions in England as it cut down on the wool trade to Flanders and the wine trade from Gascony. However, in 1340, while attempting to hinder the English army from landing, the French fleet was almost completely destroyed in the Battle of Sluys. After this, England was able to dominate the English Channel for the rest of the war, preventing French invasions.

Conflict over the succession to the Duchy of Brittany began the Breton War of Succession, in which Edward backed John of Montfort and Philip backed Charles of Blois. Action for the next few years focused around a back and forth struggle in Brittany, with the city of Vannes changing hands several times, as well as further campaigns in Gascony with mixed success for both sides

Sir William was a Commander at this battle.

Crécy was a battle in which an English army of 12,000 to 16,000 (depending on source), commanded by Edward III of England and heavily outnumbered by Philip VI of France's force of 35,000 to 100,000 (depending on source), was victorious as a result of superior weaponry and tactics, demonstrating the importance of the modern military concept of fire power. The effectiveness of the English longbow, used en masse, was proven against armoured knights, contrary to the conventional wisdom of the day which held that archers would be ineffective and be butchered when the armoured units closed in.

In the battle, the French knights, protected by mail reinforced with plate, nearly exhausted by charging several miles into the fray (against their king's wishes) and having to walk through a quagmire of mud to charge up a shallow hill into English and Welsh arrow storms, were cut down. The result was that much of the French nobility died, perhaps even a third (estimates of the actual numbers in each army vary considerably, depending on the source).

Knights' armour had not yet evolved to the stage where longbows could not penetrate, and the knights' horses were barely protected at all. The storm of arrows killed or disabled the knights' mounts, and left the knights floundering about in the mud on foot beneath a withering fire.

Just as importantly, the hired Genoese crossbowmen who were at the forefront of the battle were not able to reach the defending English with their fire partly because of the shorter range of crossbow fire, and partly because the rain slackened their crossbow strings. As the Genoese attempted to retreat, they were cut down by the French knights who considered the withdrawal to be cowardly. Thus the necessary support for the mounted knights was eradicated early in the battle, due mainly to the French themselves.

The battle is seen by many historians as marking the beginning of the end of classic chivalry; during the course of the battle, many of the prisoners and wounded were killed. This was against the chivalric codes of warfare, and knights on horseback were no longer "undefeatable" by infantry.

Crécy may also have seen the first real use of cannon on the European battlefield, which were used only in small numbers by a few states during the 1340s. "Ribaldis", a type of cannon, were first mentioned in the English Privy Wardrobe accounts during preparations for the battle between 1345 and 1346, and they were perhaps employed against both the Genoese and the cavalry.[1] Similar cannon would appear also at the Siege of Calais in the same year, although it would not be until the 1380s that the "ribaudekin" was mounted on wheels.[1] The use of firearms at this battle is only mentioned in one contemporary account of the battle, that of Villani (d. 1348). Villani did travel abroad during much of the early 14th century, yet he had returned to his home in Florence at the time of the Battle of Crécy, so his information was likely second hand if not third or fourth hand. His account also conflicts with almost all of the other contemporary chronicles of this time on the events_new_new of the battle, specifically the use of firearms. In one of the later versions of his chronicle, Froissart does mention guns being used in the battle, but by that time firearms had become more common in warfare. His earlier versions fail to include any mention of firearms. So while firearms were perhaps employed, their possible effect on the battle should be viewed critically.[2]

The political consequences of the battle were significant especially for Edward III, who had financed and supplied his expedition to Normandy with increasingly unpopular policies. The widespread use of purveyance and the arresting of ships to provide transport for his armies had left the King with potential sources of discontent in his kingdom. Likewise, the bold and unprecedented move to expand compulsory service, usually required only for defence of the coasts, to supply overseas service in France proved to be deeply unpopular with many of his subjects. However, the successes of the campaign did much to mute opposition when Parliament was convened on 13 September 1346.

English casualties were light but there were thousands of French and allied dead, among them the counts of Flanders, Alençon, and Blois, Rudolph, Duke of Lorraine, and King John of Bohemia, a French ally, father of Charles IV of the House of Luxembourg (future Holy Roman Emperor). Charles lost many of his best knights, with Charles himself escaping wounded from the field.