It is always difficult to assess Mary’s reign due to both it’s shortness and her own health troubles. But to understand Mary and the impact of her rule, you have to understand what she overcame and her ultimate triumph.

For make no mistake, Mary was a formidable Tudor; she displayed the ruthlessness of her Father and the dynastic skill of her Grandfather. She overcome incredible adversity and it is a measure of her toughness and agency, that we actually dismiss her reign so.

It could have been, SHOULD have been, much, much worse.

A nice quick summery of her reign I give at the end specifically to answer your questions, but the nature of the forum being what it is, to get to it I need to explain where I get my conclusions from.

Background to her reign:

Mary has emerged from a particularly traumatic teenage years displaying all the hallmarks of suffering from stress related illnesses. These I wrote about separately in a different answer.

Unlike her brother Edward (whose childhood had been idyllic) and her half sister Elizabeth (who had faced death all around her), Mary had been the subject of intense emotional abuse.

She entered her twenties rocked by forces beyond her control; no suitable husband could be found for her (Henry’s desire was to marry her off to some foreign Prince so he could be rid of her and his beloved Edward’s succession would be easier); Henry’s break with Rome had limited the number of Catholic’s who wanted to marry into the dynasty however, (even if Henry’s move was utterly driven by pragmatic Tudor realpolitik); and Mary would marry no Protestant.

As such she found herself placed in dangerous political grounds; when the Pilgrimage of Grace began, the rebellion against Henry’s religious reforms, one of the key demands was that Mary was returned into the line of succession.

Whereas Mary had no role whatsoever in the rebellion, she was now a figure for the opposition to Henry to rally around. Do not underestimate the danger this placed her life in. And we must credit her for her ability to remain out of it and able to be placed back in the line of succession.

From Jane Seymour onwards, whenever her father was not married, Mary was in effect First Lady of England; while she liked to gamble, sing and dance, she remained a stalwart of her father. She clearly inherited from him the understanding that while they differed in religion, the dynasty came first.

Where her skill, both politically and emotionally really kick in, is upon the death of her father. Mary is now heir to the priggish and very precocious Edward. And here is where her position becomes much more politically volatile.

We forget what the transition from Henry to his tween son meant; while he was a brutal tyrant and seemingly willing to inflict draconian methods to keep stability, he HAD kept England stable. With his passing, England immediately became highly unstable.

Henry had made sure that power in his reign was focused on him and the men HE appointed to run the country in his name. He followed his father’s tradition of making sure no other noble family could even come close to gaining real political power; he had turned against Rome but has curtailed the excesses of Protestantism as well. HE was the power.

Immediately this status quo was gone. Edward was under the control of ambitious Nobles, forming a hard line Protestant faction. You saw this instability (and Mary witnessed this close up) early, with the actions of Thomas Seymour. The brother of Edward Seymour, the Lord Protector of England, and the younger brother of the late and influential Jane Seymour, he had married the late King’s widow, Catherine Parr, finding himself now in the household which included 11 year old Jane Grey and 13 year old Princess Elizabeth.

Thomas clearly tried (or even succeeded) to groom/abuse Elizabeth, leading to Parr to send her away for her own safety; when Parr died in childbirth, Thomas had enacted a wild, hair brained plan to marry Elizabeth and place himself in his brothers position. This wild scheme failed, but saw Elizabeth face harsh questions as to her role in the plot (she had none BUT it resulted in Elizabeth losing the small circle of women who supported her for several years).

For Mary this was a powerful lesson in the dangers of her position. She had seen her own childhood governess, the Countess of Salisbury, bloodily and horrendously executed, and it was clear what few friends she had (including her beloved Susan Clarencieux, her longest serving servant and confidant) she wished to keep.

Mary was now the pivotal figure in national politics; she refused, point blank, to agree to her younger brothers demands she convert to Protestantism. This at once made her the most important figure of dissent within the Kingdom. She tried to play down her role, removing herself to her estates for much of Edward’s reign and refusing to be a public figure.

Meanwhile the weakening of the Tudor regime continued with Edward Seymour toppled and John Dudley, 1st Duke of Northumberland, now taking over as Lord Protector. His raise saw even more militancy in the Protestant faction and more moves to force Mary to renounce her Catholicism.

There were plans to smuggle her out of the country, but these came to naught; yet she was willing to have a public blazing argument with her brother (refusing point blank to continence his demand for her to recant her faith in 1550) which continued to endanger her life.

In truth her fate was somewhat sealed had not Edward suddenly been stricken by disease and died in 1553. And it is at this point that Mary reveals her staggering skill.

The attempted toppling of the dynasty:

John Dudley and his faction (who are basically the Privy Council, the government of England) decide Mary cannot be queen. They reject the Tudor line of succession (having convinced the dying Edward to remove both his sisters from it) so he can place Lady Jane Grey upon the throne to secure England as a Protestant nation.

Not since Henry VII has usurped the Plantagenets had the Tudor’s faced such a direct attack upon their dynastic claim to the throne. Mary, for all her prudishness and nervous temperament, quietly revealed the steel in her blood, the ruthless stubbornness that had seen her dynasty utterly change England.

Dudley has the dying teenager summon Mary to see him; Mary correctly guessed that this was a pretext to arrest her, so she fled to her estates in East Anglia, where there was support, if not for her personally, then certainly against the Dudley regime.

She acted swiftly and boldly upon her brothers death; she wrote to the Privy Council demanding her rights as rightful Queen, the letter arriving the same day as Jane Grey was crowned, and while the regime was wondering how to respond she was already gathering forces (including support from her half-sister Elizabeth) in Suffolk and was planning on marching in London.

Dudley’s regime collapsed; England was a Tudor kingdom and Catholic and a women it didn’t matter. The Dynasty won out and Mary claimed her throne.

For this act alone she should be credited.

Continued below