To be fair, if I had a week of preps between them making the king agree to send his forces with them against the cult in the woods and the actually woods themselves, I would be more opened to allow it to happen than if the session STARTED with this and then straight to the woods.
It might be actually the deal breaker/maker. When do they do this in the session, as arbitrary as it sounds. Do they give me time to plan this ? My answer to them will probably follow the answer to this question.