I think there are probably risks that cannot be insured in practice. One of the particulars of war is that it might destroy many insured parties at once, overwhelming any conceivable insurance syndicate. Yes, you could perhaps sell a few policies with limited coverage, but if you cannot credibly calculate the probability of the event occurring it seems hard to imagine there is a price that will make sense to both parties.

That's no different to any other catastrophe. Earthquake or tsunamis etc.

> it seems hard to imagine there is a price that will make sense to both parties.

The price you'd have to pay is likely to be extremely high due to the value at risk and level of uncertainty, and yes insurance companies would also be modelling their exposure and stop selling at some point. A single tanker, for example, could almost certainly find a price at Lloyds.