The United Nations Convention on the Rights of the Child, Article 38, (1989) proclaimed: "State parties shall take all feasible measures to ensure that persons who have not attained the age of 15 years do not take a direct part in hostilities".
Even here the UN says that 15yo and up are responsible for their actions, as they are no longer deemed children when it comes to combat.
The Geneva Convention (all 4 of them) only gives specific rights to legal combatants. "Unlawful Combatants" do not have the same rights as legal combatants. In fact, unlawful combatants have very few rights.
The International Red Cross does not dictate international law. They can propose or suggest, but they can not make it. This is why the they use the term Optional Protocol. So using them as "proof" of international law is dishonest.
Canada does not have jurisdiction over it's citizens outside of Canada. No international law allows for this. They can ask, but if the offer is refused, there is nothing they can do about it, legally anyway.
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Let's recap the story.
Kadhr, under international law, was a legal adult and responsible for his own actions.
He was an unlawful combatant. As such, his actions were classed as terrorism and war crimes.
He confessed.
As an unlawful combatant, he did not have the same legal rights as a legal combatant.
"Unlawful combatant" are also called "unprivileged combatants" in the Geneva Convention due to the fact that they have very few rights. They have their basic human rights and the right to be seen by the International Red Cross. After that, they have almost no other legal rights.
Sending Kadhr to Gitmo was completely legal under international law. The international Criminal Court, in the Celebici Judgment, set the precedent for this.
The US sent at least 2 under 15yo's back to Canada, in line with international law.
So, what we have here is a Kadhr acting in a completely illegal manner and getting punished for it. Trudeau, in secret, gives him a $10.5 million taxpayer funded payout for his troubles.
Clearly, Trudeau has more love for self confessed terrorists/war criminals than he does for their victims.