In 1055, the King of Wales Gruffydd ap Llywelyn and King of England Edward the Confessor reached agreement that would ensure peace between their two nations.
Gruffydd had worked for his entire life piecing together the warring bands of Wales. He grew up as the lazy son of King Llywelyn ap Seisyll, the ruler of both Gwynedd and Powys who had been usurped. One New Year's Eve, he was driven into the streets by his frustrated sister to find something to do. He began loitering around the kitchens, where he heard a cook muttering about a piece of meat in the stew that kept floating to the top and had to be continually pushed back down to cook. The meat became an odd inspiration that changed his life, and he dedicated himself to achievement. Gruffydd took back rule of Powys and soon stormed Gwynedd to the north. Over the years, he stitched together the Welsh kingdoms while beating back invaders such as Danes, Normans, and English, especially the Mercians just to the east.
War with England came time and again. Welsh raids infuriated English, who counterattacked readily. When Aelfgar (husband of the famed Lady Godiva) was exiled from East Anglia and formed an army to retake his earldom in 1055, Gruffydd volunteered to join him. After they had substantial success raiding Hereford, Edward the Confessor's favorite Harold Godwinson was given an army and charged to defeat the invaders. The two allies retreated, and Harold managed to end the war through diplomacy.
A tenuous peace was struck until the death of Aelfgar in 1062. Rumors spread that Harold was eager for revenge. The aging Gruffydd determined that peace had benefited his country and sued for lasting peace. Despite intrigues plotted by Harold, Edward the Confessor, himself aged, decided that the English could use a stable ally on Britain rather than a series of weak traditional kingdoms. On his deathbed in 1066, while proclaiming Harold his successor, Edward ordered him to vow never to begin unprovoked war with Wales.
The diplomatic move proved a Godsend. While Harold held claim to the throne as legal successor, other potential kings rose up from Europe. Duke William II of Normandy gained approval from the Church to seize England; Harold organized an army on the Isle of Wight to meet him, but the Norman never showed, claiming bad winds. Meanwhile, in the north, Harold's brother Tostig and the latter-day Viking Harald Hardrada invaded with victories over locals. Harold raced from London and defeated them at Stamford Bridge on September 25.
While Harold was in the north, William appeared in the south. Harold began to march his army southward, but his troops were exhausted and many of them, conscripted peasants, deserted to return home for the harvest. Although Harold felt he could defeat William on his own, his council advised calling fresh reinforcements from their ally, Gruffydd of Wales. Begrudgingly, Harold did.
At the Battle of Hastings, the Welsh were instrumental in defeating the Normans. Norman cavalry assaulted the English shield-wall time and again, which may have crumbled if not for the surprise flanking charge done by the swift Welsh troops who had been well practiced fighting Normans. William retreated to Normandy, and Norman attention would shift southward to the rest of France and the Mediterranean. Harold affirmed himself as king and became a proud defender of Gruffydd. The English and Welsh royalty would intermarry, strengthening the alliance of both countries.
The English, Welsh, and Scots would play instrumental roles in the Crusades, all the while battling each other in wars that kept Britain largely in balance. Each would take turns at attempts to conquer Ireland with varying degrees of success. While the rest of Europe later enjoyed the Renaissance, Britain would lag behind despite a surge of technical development in Scotland from the Knights Templar hosted by the excommunicated Robert the Bruce. England followed suit after the Dutch in building an overseas empire, along with temporary colonies started by the Welsh and Scottish. but these were far outpaced by those of continental Europe, especially France, Spain, and Portugal.
In the modern era, Britain enjoyed a great deal of economic growth due to its mines rich with coal and iron. As world attention left Spain and France focusing on Germany and Russia, Britain became a quiet region of Europe known for light manufacturing and tourism in its several countries.
In reality, the peace between England and Wales was far more delicate. Upon Aelfgar's death in 1062, Harold ambushed Gruffydd, who managed to escape only to be murdered by his own men. Normans under William the Conqueror would seize England and begin the process of uniting Britain by war and Parliamentary acts. Edward I of England completed the lasting conquest of Wales in 1283.
Based upon an idea originally posted on Today in Alternate History.