@Robbini It is a monarchy, after all. They don't really have to make the public believe it, they just have to spin something that whoever is in charge of the Kingdom's treasury is willing to plausibly endorse. If this hypothetical minister of the treasury is on board with the war plans, the kingdom can easily divert the funds that are supposed to be allocated to the Patriotic Hero's fiefdom, possibly to the upcoming war. It might initially be a tough sell on the public that the Great Patriotic Hero turned on them, but when he doesn't show up for the War, that sentiment can be easily turned.
I'd missed that the noblewoman was an actual Princess, though. It seems unlikely that the Kingdom would be making this offer officially without a more formal entourage. That said, not very much has been revealed about the structure of the kingdom. I'm starting to wonder if the war that's being prepared for is a Succession War.