> We're dealing with the concept of "International Law", which is largely understood as agreements / treaties amongst different countries.

Well this is true of a lot of international law, it doesn't apply here. The ICC largely deals with things that are preemptory norms which apply regardless of if you sign the treaty.

> The ICC largely deals with things that are preemptory norms which apply regardless of if you sign the treaty.

That's irrelevant. Anyone can form an independent organization and proclaim that nations of the world are subject to the rules set forth by that independent organization.

The point is that they have no intrinsic authority.

Authority comes from either moral sanction (of the people, by the people / consent of the governed) or through force.

In other words, the enforcement mechanism has to come from those that opt-in to that organization. i.e: through mutual agreement.

Which means that any "violator" nation can then say "GTFO and I dare you to come at me and see the full force of my police (if you try to arrest my citizens) or my military (if the participating nations declare war on me in an attempt to enforce these 'laws')."

So it still can only come about through mutual agreements between nations. Otherwise it is nothing more than a rogue body that sends armed thugs to try and enforce its rules while nations get to say "We neither recognize nor agree to those rules, nor do we recognize your authority to enforce them. However, you are subject to our laws while you are trying to execute your 'warrants' on our soil. And we will arrest YOU and throw you in our jails if you interfere with the rights of any one of our citizens."