I reread 'Leviathan' last month, and what sticks with me isn't the big war, but how Deryn Sharp has to perform this exhausting, constant masquerade. The plot's this alt-history WWI where the Central Powers (the Clankers) use giant walking machines, and the Allies (the Darwinists) have fabricated beasties as living airships and weapons. Aleksandar Ferdinand, a Clanker prince on the run after his parents are assassinated, and Deryn, a girl disguised as a boy serving on the British airship Leviathan, get thrown together. Their stories converge when the Leviathan crash-lands in Switzerland and Alek's group finds them.
It's less a straightforward war story and more a survival adventure that forces the two sides—and their philosophies—to cooperate. The real tension for me was always whether Deryn's secret would blow up her life aboard ship, especially as she starts to actually like Alek. The climax involves defending Istanbul from a Clanker uprising, setting up the geopolitical mess for the next book. Westerfeld's real feat is making the fabricated whale and its ecosystem feel as real and mechanical as the walkers.