In the Magna Carta, does law of the land apply to the land regardless of which sovereign controls it or only to a sovereign which signed the charter?

Question

EG if the King of France comes over and takes over Kent and for some reason the King of England agreed to a treaty saying he could have it, maybe in return for Gascony being firmly under the English Crown, would they be supposed to be bound to respect the law of the land in Kent, as they were up to the point of the takeover? Or would the law of this nature apply only to the degree say Kent was part of the dominions of the King of England?

Answer 1

would they be supposed to be bound to respect the law of the land in Kent

It would depend on the treaty, which may not say anything about it, and if it does, it may not have an effective enforcement mechanism. Aside from the treaty, the new sovereign can generally change the law of the land, subject to its own constitutional requirements, just as earlier parliaments (and sovereigns) cannot bind successive ones. The only thing that presently stands between Kent and the absolute rule of Charles III is an act of parliament receiving royal assent.

Answer 2

The law in a transferred territory is unchanged

It is an old tenet of international law that the transfer of occupied land leaves the laws and rights of citizens unchanged unless and until the new sovereign changes them; unoccupied land adopts the laws of the new sovereign immediately. Which is why Louisiana uses civil law, it brought its existing French law with it when it joined the United States.

For your example, unless and until the French government changed a law, the common law of England including all existent statutes of the UK Parliament would apply in Kent. French courts resolving disputes there would use the English adversarial system, rather than the French inquisitorial one, English procedure, juries, and apply English law.

The treaty itself may make some changes to the law and some laws might be changed pretty quickly to facilitate administration and good governance. But some can hang around for centuries. Most of the mixed systems of law throughout the world are a legacy of transfers of sovereignty, largely through colonialism.

Note that this is an early-modern doctrine and doesn’t really apply to periods before the idea of modern nation states, such as the medieval period of the Magna Carta.