Battle of Hastings
Background: In 911, the Carolingian ruler Charles the Simple allowed a group of Vikings to settle in Normandy under their leader Rollo. Their settlement proved successful, and they quickly adapted to the indigenous culture, renouncing paganism, converting to Christianity, and intermarrying with the local population. Over time, the frontiers of the duchy expanded to the west. In 1002, King Æthelred II married Emma, the sister of Richard II, Duke of Normandy. Their son Edward the Confessor spent many years in exile in Normandy, and succeeded to the English throne in 1042. This led to the establishment of a powerful Norman interest in English politics, as Edward drew heavily on his former hosts for support, bringing in Norman courtiers, soldiers, and clerics and appointing them to positions of power, particularly in the Church. Edward was childless and embroiled in conflict with the formidable Godwin, Earl of Wessex, and his sons, and he may also have encouraged Duke William of Normandy's ambitions for the English throne.

Succession crisis in England: King Edward's death on 5 January 1066 left no clear heir, and several contenders laid claim to the throne of England. Edward's immediate successor was the Earl of Wessex, Harold Godwinson, the richest and most powerful of the English aristocrats and son of Godwin, Edward's earlier opponent. Harold was elected king by the Witenagemot of England and crowned by Ealdred, the Archbishop of York, although Norman propaganda claimed that the ceremony was performed by Stigand, the uncanonically elected Archbishop of Canterbury. Harold was at once challenged by two powerful neighbouring rulers. Duke William claimed that he had been promised the throne by King Edward and that Harold had sworn agreement to this. Harald Hardrada of Norway also contested the succession. His claim to the throne was based on an agreement between his predecessor Magnus the Good and the earlier King of England Harthacnut, whereby, if either died without heir, the other would inherit both England and Norway. William and Harald Hardrada immediately set about assembling troops and ships for separate invasions.

Tostig and Hardrada's invasions: In early 1066, Harold's exiled brother Tostig Godwinson raided southeastern England with a fleet he had recruited in Flanders, later joined by other ships from Orkney. Threatened by Harold's fleet, Tostig moved north and raided in East Anglia and Lincolnshire. He was driven back to his ships by the brothers Edwin, Earl of Mercia and Morcar, Earl of Northumbria. Deserted by most of his followers, he withdrew to Scotland, where he spent the middle of the year recruiting fresh forces. Hardrada invaded northern England in early September, leading a fleet of more than 300 ships carrying perhaps 15,000 men. Hardrada's army was further augmented by the forces of Tostig, who supported the Norwegian king's bid for the throne. Advancing on York, the Norwegians occupied the city after defeating a northern English army under Edwin and Morcar on 20 September at the Battle of Fulford.

English army and Harold's preparations: The English army was organised along regional lines, with the fyrd, or local levy, serving under a local magnate – whether an earl, bishop, or sheriff. The fyrd was composed of men who owned their own land, and were equipped by their community to fulfil the king's demands for military forces.  For every five hides, or units of land nominally capable of supporting one household, one man was supposed to serve. It appears that the hundred was the main organising unit for the fyrd. As a whole, England could furnish about 14,000 men for the fyrd, when it was called out. The fyrd usually served for two months, except in emergencies. It was rare for the whole national fyrd to be called out; between 1046 and 1065 it was only done three times, in 1051, 1052, and 1065. The king also had a group of personal armsmen, known as housecarls, who formed the backbone of the royal forces. Some earls also had their own forces of housecarls. Thegns, the local landowning elites, either fought with the royal housecarls or attached themselves to the forces of an earl or other magnate. The fyrd and the housecarls both fought on foot, with the major difference between them being the housecarls' superior armour. The English army does not appear to have had a significant number of archers.
Harold had spent mid-1066 on the south coast with a large army and fleet waiting for William to invade. The bulk of his forces were militia who needed to harvest their crops, so on 8 September Harold dismissed the militia and the fleet. Learning of the Norwegian invasion he rushed north, gathering forces as he went, and took the Norwegians by surprise, defeating them at the Battle of Stamford Bridge on 25 September. Harald Hardrada and Tostig were killed, and the Norwegians suffered such great losses that only 24 of the original 300 ships were required to carry away the survivors. The English victory came at great cost, as Harold's army was left in a battered and weakened state, and far from the south.

William's preparations and landing: William assembled a large invasion fleet and an army gathered from Normandy and the rest of France, including large contingents from Brittany and Flanders. He spent almost nine months on his preparations, as he had to construct a fleet from nothing. According to some Norman chronicles, he also secured diplomatic support, although the accuracy of the reports has been a matter of historical debate. The most famous claim is that Pope Alexander II gave a papal banner as a token of support, which only appears in William of Poitiers's account, and not in more contemporary narratives. In April 1066 Halley's Comet appeared in the sky, and was widely reported throughout Europe. Contemporary accounts connected the comet's appearance with the succession crisis in England.
William mustered his forces at Saint-Valery-sur-Somme, and was ready to cross the English Channel by about 12 August. But the crossing was delayed, either because of unfavourable weather or to avoid being intercepted by the powerful English fleet. The Normans crossed to England a few days after Harold's victory over the Norwegians, following the dispersal of Harold's naval force, and landed at Pevensey in Sussex on 28 September. A few ships were blown off course and landed at Romney, where the Normans fought the local fyrd. After landing, William's forces built a wooden castle at Hastings, from which they raided the surrounding area. More fortifications were erected at Pevensey.

Norman forces at Hastings: The exact numbers and composition of William's force are unknown. A contemporary document claims that William had 776 ships, but this may be an inflated figure. Figures given by contemporary writers for the size of the army are highly exaggerated, varying from 14,000 to 150,000. Modern historians have offered a range of estimates for the size of William's forces: 7,000–8,000 men including 1,000–2,000 cavalry; 10,000–12,000 men; 10,000 men including 3,000 cavalry; or 7,500 men. The army consisted of about one half infantry, one quarter cavalry, and one quarter archers or crossbowmen. Later lists of companions of William the Conqueror are extant, but most are padded with extra names; only about 35 named individuals can be reliably identified as having been with William at Hastings.
The main armour was chainmail hauberks, usually knee-length, with slits to allow riding, some with sleeves to the elbows. Some hauberks may have been made of scales attached to a tunic, with the scales made of metal, horn or hardened leather. Headgear was usually a conical metal helmet with a vertical band guarding the bridge of the nose. Horsemen and infantry carried shields. The infantryman's shield was usually round and made of wood with metal reinforcement. Horsemen changed to a kite-shaped shield and were usually armed with a lance. The couched lance, carried tucked against the body under the right arm, was relatively new and probably not used at Hastings, as the terrain was unfavourable for long cavalry charges. Both infantry and cavalry usually fought with a straight sword, long and double-edged. The infantry could also use javelins and long spears. Some of the cavalry may have used a mace instead of a sword. Archers would have used a self bow or a crossbow, and most would not have had armour.

Harold moves south: After defeating his brother Tostig and Harald Hardrada in the north, Harold left much of his forces in the north, including Morcar and Edwin, and marched the rest of his army south to deal with the threatened Norman invasion. It is unclear when Harold learned of William's landing, but it was probably during the march. Harold stopped in London for about a week before Hastings, so it is likely that he spent about a week on his march south, averaging about 27 mi (43 km) per day, for the approximately 200 mi (320 km).