In the summer of 1422, Henry V, founder of the Plantagenet empire and one of the decisive figures in world history, was at the Château de Vincennes, near Paris, when he was struck down by dysentery. By the end of August, he had made arrangements for his brother to rule as his baby son's guardian.
But then, as one chronicler put it, "God smiled on England." The fever lifted and medieval history's hero king pulled through. A year later, he was back in the saddle and the subjugation of France went on. Had Henry been struck down at such a young age, it is plausible that the Treaty of Troyes, guaranteeing him the French crown after the death of the incumbent, would have fallen apart. It is even possible that England would have lost the Hundred Years War, unlikely as that sounds today.
In the event, Henry became the first dual king of England and France, with isolated territories in the south holding out until the 1430s. By that time, not even his infatuation with his mistress, a former prostitute from eastern France called Joan - sarcastically nicknamed "the Maid of Orleans" - could detract from his popularity and prestige.
When Henry died at the age of 60, the Anglo-French dual monarchy was established. The court had moved from London to Paris, and as a result French was confirmed as the language of the political classes - a situation that endured until the first half of the 19th century. His successor, the pious Henry VI, ruled over one of the richest, most stable societies in Europe.
By the time his son, Edward IV, succeeded to the throne in 1485, the dual monarchy was at the vanguard of the movement we know as the Renaissance, with Leonardo da Vinci just one of the many artists lured north by Edward's patronage.
It is doubtful, however, whether the dual monarchy could have lasted. By the time the Wars of Religion began in the 1580s, many English merchants had come to see the Plantagenets as remote, French autocrats. Protestantism and English popular resentment became tightly intertwined. Shakespeare even wrote a celebrated political play, Henry V, satirising the former king as a sissified Catholic fop.
It was not until 1789 that England definitively went its own way, the storming of the Tower of London paving the way for a republic under Charles James Fox.
The irony is that, even though France remains the ancestral enemy, the cultural special relationship could hardly be stronger. The beret and the polo neck remain essential components of English national dress, pétanque is still our national sport and, above all, everybody loves a mime artist. What a tragedy for England it would have been if Henry V had died young.