We know that Vizzini was paid by Humperdinck to start a war between Florin and Guilder.
What made Humperdinck want to go to war with Guilder?
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In the book, Humperdinck, along with the King, Queen and the Count were discussing how he needed to pick a bride. They agreed upon Princess Noreena of Guilder, as it would be a politically expedient match. So they invite the Princess over so Humperdinck could "have a look at her". Now, to quote the book:
When she met the Prince, she was wearing one hat, when he asked her for a stroll, she excused herself, shortly to return wearing another, equally flattering. Things went on like this throughout the day...
That night they would have a feast. The book continues.
This night was more typical than less; the winds whistled constantly and the candles constantly needed relighting, and some of the more daringly dressed ladies shivered. But Prince Humperdinck didn't seem to mind, and in Florin, if he didn't, you didn't either.
At 8:23 there seemed every chance of a lasting alliance starting between Florin and Guilder.
At 8:24 the two nations were very close to war.
So, what happened? Here is the timeline
..they (the servants) did a good enough job, considering that everything in the room was flying this way, that way, fans and scarves and hats.
Particularly the hat of Princess Noreena.
It flew off to the wall behind her, where she quickly retrieved it and put it properly on. That was at 8:23 and fifty seconds. It was too late.
At 8:23:55 Prince Humperdinck rose roaring, the veins in his thick neck etched like hemp. There were still flames in some places, and their redness reddened his already blood-filled face. He looked, as he stood there, like a barrel on fire. He then said to Princess Noreena of Guilder the five words that brought the nations to the brink.
"Madam, feel free to flee!"
And with that he stormed from the Great Hall. The time was then 8:24.
Prince Humperdinck made his angry way to the balcony above the Great Hall and stared down at the chaos. The fires were still in places flaming red, guests were pouring out through the doors and Princess Noreena, hatted and faint, was being carried by her servants far from view.
Queen Bella finally caught up with the Prince, who stormed along the balcony clearly not yet in control. "I do wish you hadn't been quite so blunt," Queen Bella said.
The Prince whirled on her. "I'm not marrying any bald princess, and that's that!"
"No one would know," Queen Bella explained. "She has hats even for sleeping."
"I would know," cried the Prince. "Did you see the candlelight reflecting off her skull?"
"But things would have been so good with Guilder," the Queen said, addressing herself half to the Prince, half to Count Rugen, who now joined them.
"Forget about Guilder. I'll conquer it sometime. I've been wanting to ever since I was a kid anyway." He approached the Queen. "People snicker behind your back when you've got a bald wife, and I can do without that, thank you. You'll just have to find someone else."
So, to sum things up. Humperdinck had always wanted to conquer Guilder, ever since he was a kid (no reason given). But, I guess you could say it was the bald princess that pushed him over the edge.