I thnk Dukes should be allowed to declare war by themselves. The Dukes of Bavaria and Swabia interfered in Italy A.D. 955, compelling the Emperor Otto to annex the Kinddom of Italy. It will also create the feeling of the Dukes not being all that loyal to the emperor.
Also, is inter-faction marriage possible? If it is, then dukes could expand their territory apart from war. For instance, if the Duke of Bavaria marries the Duke of Swabia's daughter, and if she is the Duke's only child, then when the Duke of Swabia died, Bavaria would inherit Swabia.
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