> we could only make them apply through force of arms which runs opposite to the rule of law

I'd say that these things are not opposites. Rather the rule of law is a way of tempering the power of force of arms. Look at your Nuremberg example - having won the war and enacted occupation by force of arms, the Allies were in a position where they could have simply imposed summary execution. Legalization was a stepping back from this dynamic - likely due to the amount of unilateral killing that had just gone on.

> those norms do not apply to the Russian world

The crux of the matter here is that parties on the receiving end of aggression (Ukraine, Palestine) have assented to adopting those norms. The legalistic theory is that by aggressing onto those parties, the aggressors become subject to the jurisdiction of whom they're aggressing against. I personally don't particularly like this model of international jurisdiction, but it is on solid longstanding ground. And so while I'll argue against this sort of thing at an individual level (especially where the nexus is say electronic communications), I don't feel much sympathy for leaders who end up suffering this dynamic because they decided to attack other countries.

In reality of course yes, to apply those laws requires force of arms - for there even to be something to temper in the first place. So Ukraine/Palestine assenting to those treaties is essentially asking would-be allies to defend them in some well-scoped circumstances, modulo the underlying diplomatic concerns.