On 14 September 1141, Matilda's forces were routed at Winchester and Robert of Gloucester captured, ending her period of ascendancy.
On 14 September 1141, Matilda's fortunes reversed dramatically at the Rout of Winchester. After Londoners rose against her earlier that summer and forced her to abandon plans for a coronation, Matilda had retreated to Oxford and then laid siege to Henry of Blois, Bishop of Winchester, at Winchester Castle. Stephen's wife, Matilda of Boulogne, rallied loyalist forces and besieged Matilda's army in turn, encircling and routing it as it attempted to withdraw.
In the chaotic retreat, Matilda's half-brother and chief military commander, Robert of Gloucester, was captured — a devastating loss, as he was the linchpin of her military campaign. Matilda herself narrowly escaped, reportedly fleeing on horseback, at one point said to have been carried on a horse tied like a corpse to evade capture.
The defeat at Winchester effectively ended Matilda's brief ascendancy as 'Lady of the English' and forced a prisoner exchange in which Robert of Gloucester was traded for the release of King Stephen, restoring Stephen to the throne and returning the conflict to a prolonged stalemate. This reversal illustrates the volatility of medieval civil war fortunes and marks the definitive end of Matilda's realistic chance of being crowned queen regnant herself, though her cause would ultimately triumph a decade later through her son Henry.