Yeah, from here on im just playing devils advocate, but how does that work when “the enemy” is legally part of your own populace?

Being a citizen of a polity does not exempt people from the laws of war; targeting civilians of one's own polity or an enemy polity with military action is subject to the same restrictions.

I don’t remember anything significant happening when syria used chemical weapons on their civilians.

Because no one had the stomach to enter into the Syrian Civil War to hold anyone accountable, not because it was legal. Assad still has a warrant out for his arrest for that very reason.

And the US police use chemical weapons all the time.

Chemical weapons are banned in military actions as a means of mutual disarmament. Chemical weapons in civilian and policing contexts are banned only in the context of excessive force, violation of human rights, etc.

If US police used nerve gas instead of tear gas, it would be illegal by international law.

The only reason i think devils advocate needs to be mentioned here is because, being an uncontacted tribe, its going to be impossible to get them to stick to international law or even local law.

And?

International law is clear that one side failing to adhere to international law does not absolve their opponents of the responsibility to adhere to international law. Only limited forms of retaliation are permitted in response to even total violation of international law by one side.

The Nazis performing genocide does not give the Allies permission to perform a little genocide, as a treat. The Nazis performing genocide means that after the war they get hanged. It does not change the standards of behavior expected of adherents of international law.

As such, it will be nearly impossible to identify combatants from noncombatants.

Distinction between combatants and noncombatants is made all the time in conflicts against groups which do not adhere to international law.

Thats the whole thing that upsets me here, is that there WILL be retaliation, and without subscribing to international laws, the village has no protections. If you dont identify your combatants, afaik everyone becomes a combatant. Thats why international law is so important.

That's not even close to true. That's some Israeli-level bullshit that has never stood up in any court of international law.

The closest claimed I can think of would be the GWOT Bush Administration, claiming that a failure to clearly designate combatants marked captured enemy combatants as 'unprivileged', a distinction that was not recognized in international law. Even the braindead bloodthirsty Bush admin did not attempt to argue that civilians were combatants because of the failure of enemy fighters to clearly mark themselves.

The tribe wasn’t an uncontacted tribe.

The Matsés had developed many survival techniques to avoid being detected by settlers, such as teaching their dogs not to bark when ordered, cultivating small farms in different hidden parts of the forest, and establishing shorter periods of itinerant agriculture

Nothing about women being kidnapped.

It mentions the ambush, it just doesn't specify the details.

In October 1964 in Requena province, in the east of Loreto, Peruvian lumberjacks and colonists were ambushed by a group of Matsés.

From another source, Jagua: A Journey Into Body Art From The Amazon