Before 1066, England consisted of multiple smaller kingdoms, sometimes under a single ruler, more often disunited. The famous battle of Hastings changed all that, when the Norman conquest of England brought solidity and unity to the country. Even though the Normans were foreigners, speaking French rather than the Anglo-Saxon dialects used by the commoners, they managed to give England a purpose and identity it had previously lacked.
The Norman kings of England had two main priorities: conquest or overlordship of the British Isles, and claiming the crown of France. At several moments, they seemed closer to achieving the latter goal and failing at the former, but history turned out the opposite.
In the 12th century, the Norman kings were Frenchmen in disguise. Rulers such as Henry II and Richard Lionheart did not even speak English; they preferred the language of their ancestors, the western European court language: French. Henry II was the first to seriously undertake the conquest of Wales, but he was defeated by the Welsh lords. He eventually came to terms with them, if only to restore peace on his western border as he was fighting his rebellious sons in France.
But both Henry II and his heir, Richard I, were pre-occupied with French affairs. Richard hardly visited his English lands, if at all, and set up a system of shire-reeves travelling the shires in his absence, dealing primarily with justice. These we still know as sheriffs. This decentralised the realm, but laid the foundations for the modern justice system.
After king John lost most of his French territories to Philip II Augustus, King of France, the Norman lords were forced to concentrate more on their English lands. A series of small civil wars occurred in the 13th century, the most important of which was the Baronial War, in which the barons captured king Henry III in battle and forced him to accept their ideas of a parliament, where the king's vassals could speak their mind on the king's policy. This was nowhere near the democratic organ we know it as today, but it gave English lords a voice that would otherwise have gone unheard, and helped anglify the court.
The Hundred Years War started in the 14th century; we will discuss this war in more detail in another chapter. The English armies, with a nice amount of Welshmen, Irishmen, Gascons and a variety of other troops thrown in (such as Bretons and Hainaulters) made excellent progress militarily, but their bloody war got them few friends with the French populace, and even though they defeated multiple French armies their territorial acquisitions were usually lost within a decade or two (the only exception being Calais, which held on for two centuries). The war helped make England a more united country, and provided otherwise unruly provinces, such as Wales, with suitable employment.
But the unity didn't last forever, and from the death of King Richard II (1399) to the death of King Richard III (1485), the country was frequented by civil war. In Wales, Owain Glyndwr rebelled against King Henry IV, and soon the prominent English noblemen Roger Mortimer and Henry Percy were up in arms, too. The young prince Henry, later Henry V, was taught the art of war in the Welsh campaigns, eventually defeating Percy at the Battle of Shrewsbury (1403). When he became king in 1413, he proved popular, as his campaigns in France testify. The immortal victor of Azincourt (1415), the most famous battle of the Hundred Years War, his reign was a triumph already. His early death, however, brought back unrest, as his mad son was incapable of governing; the War of the Roses had started.