I have a simple two-part plan for fixing this problem.
Part 1
Cap the total number of countries that can enter the war through cascade per DoW. 4-6 should be sufficient.
Part 2
Force any nation not in direct alliance with the original defender to pass a Relations check with an increasing penalty for each step removed from direct alliance.
For example,
Rome declares war on Macedonia. Macedonia's ally Pontus may enter the war in the normal fashion. Pontus's ally Bosporan Kingdom must have Relations => 125 with Macedonia in order to enter the cascade. If BK enters the war, BK's ally Sparta must have Relations => 150 with Macedonia to enter through cascade. And so on.
Something like this would counter the argument that alliance cascade makes diplomacy more important, since such a fix makes diplomacy even more important than raw alliance cascade. A nation with few allies but good Relations with everyone could have many nations rushing to its defense, but a hated nation would have to rely on its direct allies alone. And of course a nation with a habit of repeatedly declaring war could still end up with lots and lots of enemies.
Part 1
Cap the total number of countries that can enter the war through cascade per DoW. 4-6 should be sufficient.
Part 2
Force any nation not in direct alliance with the original defender to pass a Relations check with an increasing penalty for each step removed from direct alliance.
For example,
Rome declares war on Macedonia. Macedonia's ally Pontus may enter the war in the normal fashion. Pontus's ally Bosporan Kingdom must have Relations => 125 with Macedonia in order to enter the cascade. If BK enters the war, BK's ally Sparta must have Relations => 150 with Macedonia to enter through cascade. And so on.
Something like this would counter the argument that alliance cascade makes diplomacy more important, since such a fix makes diplomacy even more important than raw alliance cascade. A nation with few allies but good Relations with everyone could have many nations rushing to its defense, but a hated nation would have to rely on its direct allies alone. And of course a nation with a habit of repeatedly declaring war could still end up with lots and lots of enemies.