Historical note for the SGA fanfic, On the Wings of an Eagle.
(If you've come here by googling for you history assignments, please go elsewhere instead. This is vastly oversimplified for the purposes of providing a very quick intro. to an AU fanfic)

Henry VIII died in 1547, and was succeeded by his young son, Edward VI. Since Edward was still a child, actual rule was in the hands of a Protector, his uncle, Edward Seymour, Duke of Somerset. Edward VI had been brought up as a Protestant, and during the six years of his reign, England rapidly moved towards being a Protestant country.

After a few years, Protector Somerset was discredited and kicked out, and John Dudley, Earl of Warwick (soon to become the Duke of Northumberland) soon took over his position. Edward VI died in 1553, but just before the king's death, Northumberland and his allies persuaded him to name his (Protestant) cousin, Lady Jane Grey (hastily married to Northumberland's son), as his heir, instead of his (Catholic) older sister, Mary.

Lady Jane Grey lasted all of nine days as queen before being toppled and replaced by Mary. Mary started undoing as many of the religious changes as she could. She married Philip of Spain, and started burning prominent Protestants who refused to recant.

In 1554, a Protestant rebellion tried to depose Mary and replace her with her sister Elizabeth, but it was thwarted. Elizabeth only narrowly escaped execution, and at the time of this story, is imprisoned at Woodstock, her future uncertain.