One of my favourite fantasy books back in the day. Thirrin, at age 14(?) becomes the warrior queen of the Icemark, and has to find allies and prepare for war and invasion. Thirrin is far from perfect, but ferocious and amazing. Her friends and allies are amazing, both the magical ones (wizards, giant talking snow leopards) and the natural ones. It takes all of her skill to organise a big retreat and then an assault as the ice thaws.
Plenty of logistics, but also plenty of personal development and bravery. An underappreciated gem.