However, Wales had already been conquered by England in the 13th century. Originally ruled by the Romans in its early history, England and Wales were seen as a single unit. By the time the Romans left Britain, Wales became an independent state with their own system of law.
However, during the Norman invasion of Wales in the 11th Century, English law was practised in certain areas of the country. By 1283, under the leadership of Prince Edward, the biggest army ever seen in England invaded and conquered Wales, bringing it under English rule.
Therefore, a few hundred years later when England was to be joined in the union with Scotland and Ireland, Wales was already part of "England", and thus was not represented in the new union's flag.