> The US should be prosecuting our own war criminals. Unfortunately, we haven't been doing a good job of it.
That is precisely the point of the ICC: to prosecute individuals who are deemed by the consensus of its member-nations as "war criminals" (where that term has a strict definition—going back to the Geneva convention—that everyone does seem to agree on when it's someone else committing these crimes against them), but where that so-deemed war-criminal's country of citizenship refuses to prosecute them, or to even recognize them as a war criminal.
> How many EU war criminals have the EU or ICC prosecuted? None.
Well, again, the ICC only exists to prosecute cases where the country of citizenship of the accused refuses to prosecute them. The individual EU member-nations have, AFAIK, prosecuted all their own war criminals, so the ICC never had to get involved.
I would also point out that the countries in "the commonwealth, the EU, and the Nordic countries" have the longest combined history of committing war-crimes (all basically against one-another)—and thus also have the most "institutional knowledge" of game-theoretic problems in getting recognized war criminals dealt with by their country-of-citizenship. This set of countries has the most reason to want some sort of moderator that will ensure that they can hold one-another accountable for their future actions (by, essentially, pre-committing to holding themselves accountable for their own future actions, even in cases where their own future governments refuse to be held so accountable.)
The countries that declined membership in the ICC are mostly (by count) the "authoritarian countries" mentioned above. Presumably, they mostly declined membership because they have at least some "war criminals" as citizens who they refuse to prosecute, who the ICC would demand they hand over.
But maybe that's not the whole of the reason. Looking at the largest-by-population countries that declined membership (or later rescinded an existing membership) we have the US, China, Russia, and India. Well, here's the story for Russia:
> Russia signed the Rome Statute in 2000. On 14 November 2016 the ICC published a report on its preliminary investigation of the Russian military intervention in Ukraine which found that "the situation within the territory of Crimea and Sevastopol factually amounts to an on-going state of occupation" and that "information, such as reported shelling by both States of military positions of the other, and the detention of Russian military personnel by Ukraine, and vice-versa, points to direct military engagement between Russian armed forces and Ukrainian government forces that would suggest the existence of an international armed conflict in the context of armed hostilities in eastern Ukraine". In response, a presidential decree by Russian President Vladimir Putin approved "sending the Secretary General of the United Nations notice of the intention of the Russian Federation to no longer be a party to the Rome Statute". Formal notice was given on 30 November.
In other words, the ICC made an official statement, and that statement was the one that most countries believe, instead of the Russian-propaganda line on the matter. By continuing to be a member after such a statement was given, Russia would be implicitly endorsing those statements, and thus weakening their own propaganda. Thus, they had to rescind their membership, as the first step to painting the ICC as corrupt or confused or whatever is required to patch the hole in the propaganda campaign.
Maybe some of the other, smaller nations saw this sort of thing coming, and this is why they didn't bother with membership in the first place. They didn't want to have to attempt to whitewash the official statements of an organization to which they were a member.
This also makes for a good explanation for China, given their current approach with the, err, "states" touching them.
I'm not sure either explanation fits the US, so much as the US just 1. hates the idea of the Continent telling it what to do, and 2. doesn't want to be interrupted from an active operation by having one of its active assets—who happens to also be a war-criminal—commandeered to face trial. (This seems a uniquely American stance, to me. Most other nations are quick to punish their war-criminal officers, even if doing so is a setback for any active plans of theirs.)