Weren't they supposed to marry, not war?

that's just a myth, there even is a black book on them (in German). one of the many examples is the Magnate Conspiracy, from WP: "Petar Zrinski and Fran Krsto Frankopan (Francesco Cristoforo Frangipani) were ordered to the Emperor's Court. The note said that, as they had ceased their rebellion and had repented soon enough, they would be given mercy from the Emperor if they would plead for it. They were arrested the moment they arrived in Vienna, and put on trial. They were held in Wiener Neustadt and beheaded on April 30, 1671."