If that's the case, Tonio probably shouldn't have had to peace anyway. But I think you exagerate his abilities in that game. Could he have retaken any prov he wanted if the Pru-Rus alliance made a deliberate effort to prevent him from taking 1-2 certain provs? I don't think so. Tonio's mobility allowed him to attack remote areas because Pru and Rus couldn't defend all their conquered territory. It doesn't mean they couldn't hold onto successfully a certain area. If they had tried, that would've been easy.
I think the real problem with this idea is that it can be abused by the attacker. What if you as France overrun half of Austria and prevent Austria from taking back any of its provs in that half of its country for the specified number of years? It automatically must cede all that land? Or what if Spain holds the Netherlands, and France overruns all of the Netherlands. That's a big chunk of land. Should Spain have to cede ALL OF IT in one war if it can't successfully defend it? Assuming that it also can't capture territory elsewhere of course.
There woud need to be limitations to this rule. Perhaps a prov limit would be best. Maybe up to three European provs? Colonial provs are different, they should be exchanged more freely, so perhaps no rule for colonial provs(except you can only take what you can stabhit for?).
The other difficulty would be explaining this rule to every sub you have, but there are worse rules out there I suppose
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I think the real problem with this idea is that it can be abused by the attacker. What if you as France overrun half of Austria and prevent Austria from taking back any of its provs in that half of its country for the specified number of years? It automatically must cede all that land? Or what if Spain holds the Netherlands, and France overruns all of the Netherlands. That's a big chunk of land. Should Spain have to cede ALL OF IT in one war if it can't successfully defend it? Assuming that it also can't capture territory elsewhere of course.
There woud need to be limitations to this rule. Perhaps a prov limit would be best. Maybe up to three European provs? Colonial provs are different, they should be exchanged more freely, so perhaps no rule for colonial provs(except you can only take what you can stabhit for?).
The other difficulty would be explaining this rule to every sub you have, but there are worse rules out there I suppose