The Magna Carta.

[Stanislaus]

@Nava

I But it’s not the first time a monarch’s power was limited by law.

Indeed. There’s a lot of myth-making about Magna Carta, which has generally been a good thing as the myths have helped push forward the argument for rights and the rule of law. But it wasn’t, at the time, all that it’s now cracked up to be.

This speechby Lord Sumption (who is both a Supreme Court Justice and a medieval historian of note, so well worth listening to on this topic) lays out the historical context of Magna Carta very nicely:

The first thing that we need to do, if we are to appreciate the historical significance of Magna Carta, is to understand the world in which it was created. Contrary to common belief, the middle ages was not an age of absolute monarchy, either before or after Magna Carta. It never could have been. Medieval societies generated very small surpluses over and above the basic cost of subsistence. Governments had limited resources of money and manpower. They could not govern without the tacit support of their subjects, and the active support of at least the most powerful of them. Medieval governments depended for their survival on an unstable mixture of sentiment, legitimacy and bluff, in the same way that most governments did until comparatively modern times. This meant that kings could not afford to act in a way that defied the contemporary consensus about how a king should behave. One of the key elements of that consensus was that the King should act in accordance with law.

[snip]

So, it is true that Magna Carta stands for the rule of law. But it is not true that Magna Carta was the origin of the principle. The English Kings had doubtless broken the law quite frequently before Magna Carta, and they continued to break it afterwards. But the idea that the King was subject to law had for a very long time been part of the orthodoxy of medieval constitutional thought both in England and elsewhere. The barons did not invent it at Runnymede. Their object was to define what the law was. No one doubted that whatever it was, the King was subject to it.

He also points out the astonishing weakness - in theory and in practice - of the famous Clause 39: "No free man shall be seized or imprisoned, or stripped of his rights or possessions, or outlawed or exiled, or deprived of his standing in any way, nor will we proceed with force against him, or send others to do so, except by the lawful judgment of his equals or by the law of the land.

The bolded bit guts everything that goes before. In pre-Parliamentary times, the law was decreed by the king. So Magna Carta would stop him imprisoning, exiling, or using force against (etc. etc.) any one - unless of course, he decreed that he could do just that. He could decree a general law permitting these actions, or target an individual via Bill of Attainder. In any case, he could show that he had followed the law of the land. The protection afforded by Magna Carta was literally parchment thin.