You can even become your own kingdom (see california, Hawaii, texas, ...) before becoming part of another kingdom.
It may not be straightforward, however; as Linebarger states:
> Formally, war may be defined as the "reciprocal application of violence by public, armed bodies."
> If it is not reciprocal, it is not war, the killing of persons who do not defend themselves is not war, but slaughter, massacre, or punishment.
> If the bodies involved are not public, their violence is not war. Even our enemies in World War II were relatively careful about this distinction, because they did not know how soon or easily a violation of the rules might be scored against them. To be public, the combatants need not be legal—that is, constitutionally set up; it suffices, according to international usage, for the fighters to have a reasonable minimum of numbers, some kind of identification, and a purpose which is political. If you shoot your neighbor, you will be committing mere murder; but if you gather twenty or thirty friends, together, tie a red handkerchief around the left arm of each man, announce that you are out to overthrow the government of the United States, and then shoot your neighbor as a counterrevolutionary impediment to the new order of things, you can have the satisfaction of having waged war. (In practical terms, this means that you will be put to death for treason and rebellion, not merely for murder.)
> ...
Note that this advice was from the mid-XX; in the XXI not all kingdoms seem to recognise the Geneva Conventions anymore!
These days it's probably a case of conjugating irregular verbs?
I am a (dissident turned) freedom fighter
You are a (perfidious) combatant
They are (drug-running) terrorists
Sadly we, the "good guys", created a dangerous precedent in the balkans when Kosovo unilaterally split from Serbia, under foreign (NATO) occupation moreover.
International law does not promote nor support unilateral secessions. If a region or autonomous republic wants to secede it should only do so in accordance to the host country laws. E.g. the Quebec and Scotland referendums were made in accordance to the host countries of Canada and UK.
But then we created that dangerous case where now every region can secede from their host one unilaterally, even if it's occupied by foreign forces. And in practice, the "legality" of it, really depends on international recognition and the undergoing narratives.
International laws have always been pleasantries, as there's no real ways to enforce them, but there were powerful incentives for everybody to play by the rules.
It's hardly a precedent, probably half of the countries worldwide have been formed by seceding from some other country against its will. U.S. would be in this half.
It's the first country to do so under foreign military presence since UN inception.
The only precedents of unilateral secession were Slovenia and Croatia from Yugoslavia and Bangladesh from Pakistan but none did so under foreign military presence.
All of the Arab countries have basically seceded from Ottoman empire under an occupation of this or that European country.
"Unilaterally" is not easy to define. Sometimes there is a long and violent struggle for independence and the metropole eventually gives in and signs some paper, sometimes it is stubborn and doesn't sign anything - the difference is not that important in my opinion.
It may not be straightforward, however; as Linebarger states:
> Formally, war may be defined as the "reciprocal application of violence by public, armed bodies."
> If it is not reciprocal, it is not war, the killing of persons who do not defend themselves is not war, but slaughter, massacre, or punishment.
> If the bodies involved are not public, their violence is not war. Even our enemies in World War II were relatively careful about this distinction, because they did not know how soon or easily a violation of the rules might be scored against them. To be public, the combatants need not be legal—that is, constitutionally set up; it suffices, according to international usage, for the fighters to have a reasonable minimum of numbers, some kind of identification, and a purpose which is political. If you shoot your neighbor, you will be committing mere murder; but if you gather twenty or thirty friends, together, tie a red handkerchief around the left arm of each man, announce that you are out to overthrow the government of the United States, and then shoot your neighbor as a counterrevolutionary impediment to the new order of things, you can have the satisfaction of having waged war. (In practical terms, this means that you will be put to death for treason and rebellion, not merely for murder.)
> ...
Note that this advice was from the mid-XX; in the XXI not all kingdoms seem to recognise the Geneva Conventions anymore!
These days it's probably a case of conjugating irregular verbs?