The feud between Florin and Guilder goes back for centuries, with even Sicilian laymen acknowledging the two as sworn enemies. While there is no agreement on the exact slight that originated the feud, each generation hostilities escalated, with passions igniting as young reagents, either eager to consolidate their claim to the throne or wary of too many rich, landed, but bored knights in their holdings, would ride off to battle in search for glory.

When Prince Humperdink, first of his name, began planning his ascension to the throne, he first sought out a beautiful peasant woman who would win the hearts of his people. As the newly revealed diary of the deceased Count Rugen clearly shows, Prince Humperdink planned to frame his sworn enemies for her murder as a prelude to war. His long term plan appeared to be consolidation of the two kingdoms into one grand empire. His hegemony fully established, he could tackle the pirate problem, urbanize the Fire Swamp, and finally exterminate the R.O.U.S. (whose constant presence in his dreams had haunted him from childhood).

Of course, his plans went unrealized when the preeminent pirate of the day utilized strategic intel, a tiny international force, and a wrestling giant, to gain access to the castle. Count Rugen perished in the Battle for the Castle, the would-be princess made her escape, and Prince Humperdink never regained his former glory, dying a broken and humiliated man.

Sources:

The Princess Bride S. Morgenstern

Would the Real Dread Pirate Roberts Please Stand Up?: New Insights into the Golden Age of Piracy

An Idiot's Guide to the Fire Swamp