A solution to the "AI either throwing everything in or standing idly by while Persia chews its arm off" problem would be to have the AI be amenable to a white peace if its invasion force of a small country got defeated, but to respond with all its resources if the small country then invades and takes one of its provinces.
This shows a proportional response where the AI is capable of two tiers of response, the first one being "Lets send our B-team in to defeat this Persia, they're not very tough and we need our main armies to keep an eye on the west... oh, bollocks, we got beaten. Better not throw good after bad or open our flanks to Prussian and Austrian invasion. sign a truce and we'll get them next time when Napoleon isn't breathing down our necks" and the second one being "Oh, bollocks, we got beaten and now they're coming for us, mobilise the armies because the Persian Shah is almost at Sevastopol".
Still, if Prussia, Austria and Sweden are engaged in war then the Russians should go straight to the "send everyone to crush the Persians" stage; they might as well because everyone they normally point the guns at is engaged elsewhere.
This shows a proportional response where the AI is capable of two tiers of response, the first one being "Lets send our B-team in to defeat this Persia, they're not very tough and we need our main armies to keep an eye on the west... oh, bollocks, we got beaten. Better not throw good after bad or open our flanks to Prussian and Austrian invasion. sign a truce and we'll get them next time when Napoleon isn't breathing down our necks" and the second one being "Oh, bollocks, we got beaten and now they're coming for us, mobilise the armies because the Persian Shah is almost at Sevastopol".
Still, if Prussia, Austria and Sweden are engaged in war then the Russians should go straight to the "send everyone to crush the Persians" stage; they might as well because everyone they normally point the guns at is engaged elsewhere.